It's Unreal
Last month, after playing a lot of Guildwars, I was in the mood for something different. So I fired up my copy of Unreal Tournament 2004 and played a whole lot of Onslaught online.
I was surprise to see that there are still a lot of UT 2004 servers out there. However, I almost always play on the same servers, the Omnip)o(tentS. Founded on March 25th 2004, Omnip)o(tentS is a clan of skilled and mature gamers (description taken from their website). The ambiance on the servers is nice and friendly, which is always refreshing after my past experiences with the Halo community.
If your looking to play UT 2004 online, I highly suggest you consider their servers. Its a good play to get that UT 2004 fix until UT 2007 comes out :-)