I more or less promised Derf a list of cool oldie games based on the GoG catalogue, and that I'd mail it to him, but I figured I could write it down on CK just as well for everyone to see. So here we go!
RPG's
A first-person RPG in the style of the Ultima games of old. Possibly one of the most atmospheric games I have ever played, due to it being set in an underground word. The surface world has fallen into a permanent darkness and ice age, so civilizations retreated underworld. The spell-casting system is innovating but also hard to master (you have to draw symbols in the air with your mouse). Basically you can combine basic spell elements into different spells, almost intuitively. 'missile' + 'fire' gives you a fire ball, 'augment' + 'life' heals you, 'time' + 'lower' + 'space' will slow time around you in a certain area for a while, and so on. Very cool. The game doesn't hold your hand on what to do so you have to figure out a lot on your own story-wise, but I can imagine some of you like that as much as I do.
Spell-casting: http://www.youtube.com/watch?v=LueGiUasvFk
Intro with short history: http://www.youtube.com/watch?v=F-AuXcVbW5U
'Only' the best RPG (story-wise) of the Black Isle golden age (with BG saga, IWD, Fallout). This game is special in that it asks so many important questions on the nature of a man, and the way those answers are filled in is largely dependent of your actions. Lots of dialogue, all well-written, interesting characters, and beautifully strange world. There are high-res mods out for this game as well as for all the other IE games of that time, which make the game even more appealing.
Pretty bugged when it first came out but very underrated. It's story isn't all that, but tactically this is the best game of its era by far. If you're looking for interesting and challenging party-based strategy in an RPG setting, this is your game. You'll have to get used to the 3.5 AD&D rules, but trust me, it's worth it. High-res mods should be available for this too I'm sure.
This is a toughie: I wouldn't exactly put it among the BEST games, but it's a personal darling. For all its faults, Arcanum offers an interesting setting of a world in which magic and medieval warfare is clashing with an industrial revolution. Character creation and customisation is massive, with many paths to follow (main 3-4 would be magic, mechanics, melee/fire arms combat, stealth, persuasion) but no restriction on blending skills from either discipline other than your personal stats and abilities. Many races with their own bonuses and disadvantages too. The game can become unbalanced towards the end because the different disciplines aren't balanced out against each other too well, but it's worth it imho. Good story, wonderful music, fascinating world and character development...
http://www.epicgames.ru/wp-content/uplo ... anum-3.jpg
http://www.rpgamer.com/games/other/pc/a ... canum1.jpg
http://www.mobygames.com/images/shots/l ... antown.jpg
Music: http://www.youtube.com/watch?v=ftvpi-6Z6qw
Adventure
One of my favourite games of all time. Blends first-person action with adventure and puzzling, has an amazing atmosphere, and the narrative deals with occultism, religious symbolism and mythology. The game is linked together by FMV but it's actually well-done. The inventory is amazing too in that it gives you a rendered version of every item in it, and even allows you to reflect on certain items, places and persons. Or even concepts. You can even ask a companion's opinion! A game that is horribly, cruelly underrated.
The Gabriel Knight Series
Consists of three games: , , and . The first two are the 'real' classics, but part 3 is still a great game. Basically they're pretty standard point-and-click adventures, but OH SO GOOD. Seriously, if you like p&c adventure games, make it these games.
Again, one of my favourite games of all time. Beautifully written, incredible expansive, with gorgeous backgrounds. The sequel isn't on GoG I think, but if you like this one you should definitely try that one too. It's on steam too. There's not really more to say about it other than that you should have played it at least once in your life, imho.
If you like murder investigations and playing inspector and all that, this is a ton of fun. Not an extraordinary game in on the whole, but definitely fun.
One of the best adventure games ever, imho. A psychological horror adventure that stays within the lines of a p&c adventure game but in such a twisted, bizarre manner that you're constantly going 'awesome'. So weird, but so good.
The Tex Murphy Games
There are a few again: , , and . (there are actually even earlier games but I'm disregarding those here) Basically it's a hard-boiled detective adventure game in a Film Noir style, with tons of FMV and pretty good acting (and jokes). The third game is the best of the series imo, but they're all really good. Again, if you think you might be into this sort of thing - investigating, being a PI, etc - lots and lots of fun.
Strategy
Age of Wonders 1 is great too, but 2 basically makes the game perfect. There hasn't been a finer fantasy TBS game since this one - not that is JUST says something about how good this is, since we haven't had many of those lately. But it is really, really good. Nothing but recommendations for it.
A mixed bag. The idea is great: you are an evil genius (bond-style) and build your evil lair in the side of a mountain - including Doomsdag Device - while you try to stay under the scanner of the international super powers who may send their agents to check out and/or destroy your operation. The artwork and 'feel' of the game is just right, so right. BUT...it has questionable design when it comes to managing your fortress. A lot of it has to be done by hand when it could easily have been AI-managed, leaving you to micromanage so much stuff that it can sometimes feel more like a chore. Now I make it sound worse than it is - it's a good game, especially if you fancy yourself an evil genius. But not without flaws.
I realise the games look like shit now, but believe me when I say they are some of the most in-depth and rewarding strategy games ever made. It's TBS though, so make sure you're into that sort of thing.
It may look a bit clunky, but it's actually an RTS, one of the finest ever at that. The scope this game had at that time was insane - really huge maps with 3d (!) units and massive, massive unit type choices - pretty much going against all RTS wisdom of its time but with great success.
The title of this game says it all - you manage a business, and it is your job to crush all competition.
It's basically an economy-based strategy title, and let me tell you, there's not much that can top the feeling of raising prices to insane levels on some silly products or undercutting your competitors. Good stuff.Shooters
Based on the comic book series (and the series/movie I think), although vert good this isn't the best shooter ever, but it's a remarkable one because of the makers' choice to do the entire game in cell-shaded comic book like graphics, giving it a really unique look. Nothing like throwing a knife and having it says 'shoosh' for a moment!
An unabashed, awesome and crazy shooter of the Duke Nukem 3D era (slightly more advanced). This game will crack you up, if not for the hilarious rednecks you're controlling (accents rock) for the crazy aliens stealing your prized pigs. Or the fact that you could regain health or boosts from stuff like beer, moonshine and pork rinds but then would start farting (hampering your movement) or become really woozy. Music is great too. A classic!
The thing with Postal 2 is...it's not a great game. It's not even a game in the sense that most of your objectives are really lame. Get milk, get autograph from Gary Coleman, etc. But...the game throws so much human stupidity at you that it basically invites you to, well, go postal.
The level of needless violence and sadism in this game can be extreme depending on your dispostion (putting out burning people by peeing on them, using a cat as a silence on a shotgun), but for those psychoes among us, this can be a very relaxing game indeed. Derf, looking at you. 
Again in the style of 3D Realms shooters of the Redneck Rampage and Duke 3D era, but this time in a sort of horror setting. You're Caleb, a sort of vampire or zombie who takes on the undead / cultist army of Chernobog...some nice finds in the game, and good action, but I prefered Redneck Rampage.
Action games
It's actually more a mix of an action game and an adventure game, but hey. It's a pretty unique game in that it does so many different things, it's not just one style of combat but you'll also be riding machines and doing many other things like puzzles and talking to people. The style of the game is amazing and was provided by Michel Ancel, a monument in gaming (graphical) design. BG&E is one of the old games that holds up the best today from almost everything I posted here, which is an achievement in itself. Definite classic.
This is game is more fun than awesome. It's more or less a series of battles you have to face as a sword-wielding soldier, with the possibility of chopping off body parts (and having them chopped off). It's all very Python-esque and often downright hilarious, but the controls can be clusmy. Your call.
If you like Tamagotchi style games, where you nurture a complex creature to adulthood - look no further. Extremely fun.
Another one of my all-time favourites. This is, without a doubt, the most brilliant, funny and stylish 3D platformer of all time. There is no comparing. You're Raz, a young kid at a camp for children with psychic abilities, but while you're there things start going terribly wrong and you start investigating. The main lure of the game is that you can 'dive' into some people's minds to solve problems there or find clues, and every mind is so radically different. A cool German agent will have a very ordered mind at first glance, but appears to have a lot of repressed memories...a milkman has insane paranoid dreams of a twisted suburbia populated with secret agents, a washed up opera singer will have nightmares about being crushed by a critic during a performance etc. It's absolutely brilliant.
Well...that's about it for now. Note that I did not include really obvious classics like the Commandos series, BG series, Jagged Alliance, IWD, Fallout, etc etc. There are also games that I know are supposed to be good but that I have never played myself, notably the Zork games, Earth 2160, Sacrifice, the Betrayal at Krondor/Antara games, Stonekeep, Kingpin, Unreal 2, Giants: Citizen Kabuto or Outcast.
