leomund: So my wife and I had a baby a few months ago, and as much as I love gaming, I am finding it impossible to get more than a half hour block of time. It makes it tough to really get into games with lots of quests, dialogue since you pretty much forget what was going on and lose the "immersion". I think what I need is something like UT2004 (maybe I'll even get it again since I lost my play discs) which I used to just play against the bots for a quick reality escape. My dream game would be some kind of an arena where you might be able to add on equipment / abilities as you won duels that don't take too long. I never followed shooter games too much being more of a TBS guy so maybe I missed something - is there anything out there like that? thanks
For shooters, I agree with the others saying
Painkiller is a great game, with an outstanding atmosphere. There aren't some generic sci-fi levels coming after each other in a generic sequence, but very different scenes with an astonishing attention to details (some examples: an opera house with stage, balconies, backstage, a huge chandelier, marble steps and so on; a train station with corridors (including vending machines and ATMs), lobbies, trains, train schedules, "Wilkommen in Zurich" boards etc.; a dock, complete with cargo trains, cranes, loading bays, crates...). It can be quite time-consuming, though, with levels requiring 20-45 minutes to complete (not counting the time you spend redoing a part of the level several times because of some tough mosnters). However, you can save anytime you want, so maybe that's not a big problem.
Nexuiz is also a worthy candidate, where you can just jump in with a couple of bots for some mindless carnage. Also, it's free.
If you wish to heed bansama's advice, I suggest you buy
Sacred Gold. It's an almost completely freeform Action-RPG (with moster scaling to ensure you can revisit areas you've already explored), which you can literally play for just some minutes or whole hours on end.