I'm curious about this too. Especially things that run on Linux. Not so much to try and improve at UrT but just to give me a break from improving at UrT
Warsow (
http://www.warsow.net/) is a very faced-paced FPS with cool graphics. I haven't played it in awhile but I remember it was a lot of fun. (Stop right there! I'm not advertising or recruiting. I'm just answering his question. PlsNoEdit.)
If you want to improve at UrT the best thing you can do is play with people that are better than you and don't let yourself use excuses like "unhit", "lag", "hacks", etc. Spec/demo, ask questions (thoughtful questions and expect rejection), try and do the things they're doing differently than you and see how they feel/effect your game. If you only play in a certain circle of players (say, exclusively on 1UP servers) it might help to branch out and try playing on some other servers. I'm sure 1UP has some great players but within any group there is going to be a balance point of skill that the group revolves around.
If you get your butt kicked on another server it doesn't necessarily mean that server is full of unhit-lag-cheating scum. Toughen up there and you might find yourself moving up the skill ladder back at home (1UP)
I'm not saying don't play on 1UP. You have a strong community here. Cherish it
I'm just speaking from my own experience playing in one circle. In that case we had an admin that kept the skill-ceiling artificially low (too skilled and talked? banned for attitude. too skilled and not talking? banned for hacks) so he could feel like a pro. Maybe that doesn't happen naturally and I don't have any reason to think 1UP's servers suffer the same way. Maybe one tiny reason but that thread is history now.
tl;dr: Improve at UrT? Play more, play smarter, play with people that make it hard. Don't forget to relax and just play, too!