I'm still addicted to UO but about to give Mortal Online a try. It's a year old now and plays much like early UO apparently, in that everywhere is PvP with Free for all loot and guard zones/criminal flagging etc. The skills work similarly to UO (no levels and classes, just use a skill to increase it) and can be learnt/unlearnt as you go to avoid constantly deleting or recreating characters.
The thing that really interests me is that the animal taming (the bit I'm most addicted to in UO) is seemingly very in-depth. You can basically breed horses you have to get extremes, such as incredibly strong or agile mounts, and also breed different coats (such as zebra!). I haven't really read up enough on it to give anything in depth (kinda don't want to, would rather just discover it as I play) but once I get on there and have given it a good run I'll post a little review here. If anyone else wants to give it a go there's a 14 day free trial. Just head to
www.mortalonline.com