I compiled a list of a few good HL2 mods for Derf, or anyone interested. I stuck to singleplayer campaigns, but there are a ton (interesting) MP mods too, like a WWI mod etc. If you want info on those just shout.

Minerva
Research and Development
Provenance
Rock 24
GoldenEye: Source

of course there's also that mod (Obsidian Conflict) that allows us to play through HL2, Ep1 and Ep 2 in co-op.