My time lately has been going into the
Dystopia mod for HL2. They just released "v1" after a long time in public beta, and shortly thereafter the requisite "v1.1" to fix the glaring bugs.
It's an extremely ambitious mod project with high production values.
It's currently plagued with some balance issues (some common to every asymmetrical team shooter I've ever seen, others unique to this particular blend of futuristic weapons) and a wave of confused noobs. The game seems in some ways designed to discourage the casual player. If any of counterstrike's popularity can be attributed to the ease with which a player may jump in and out of games, and the shortness of rounds, then dystopia is disadvantaged. Rounds are long assault/defense games involving an often tangled web of buttons both in "meatspace" and "cyberspace" (see the laserium imaged above) and frequently the majority of the players stay for the duration.
I've often found the communities around smaller mods to be some of the best (gloomq2, I'm looking wistfully at you) and the global ranking system, although more of a global XP system rewarding play-time, does an interesting job of promoting persistence across servers and community cohesiveness.
Ultimately the gameplay is a little bit like Tribes, but a little more like Unreal 2's XMP. The major changes in v1 make the game a bit more polished, while adding a few new difficult-to-balance weapons. The implant system is also expanded, seemingly to confound complacency and sow chaos. For example the long-standing war between the stealth implant and the infrared implant was made more complicated with the "coldsuit" -- which renders one invisible in infrared, but cannot be combined with stealth -- and with the addition of glowing red eyes to make those using infrared more visible.
Dystopia makes excellent use of the Source engine, and requires a bit more horsepower than vanilla to keep framerates acceptably high.