Darkfall Online is, at the moment, the only real successor to Ultimate Online. It's as close as you'll get. Its an indie game that had a TON of hype behind it, which of course only ends up destroying an MMO's launch as it did with Darkfall.
The game survived the not so good launch and has been building momentum for months now and the community behind it is great. Big things are coming for this game.
Anyways its a skill based, free for all full loot system with a player run economy, fps like combat (not auto target), and player built cities.
I believe they just started a 14 day trial.