Half-Life 2: Deathmatch
I run 4 of my own servers running mainly my own custom maps, co-located at Telehouse, Docklands and we have a fun clan, The Sanctuary UK.
http://www.thesanctuaryuk.co.uk/sanctuaryuk.htm
We have lots of members who have become friends, and we enjoy playing the game for a laugh, and we hold Arena and Last Team Steam nights on Friday evenings using our HL2DM Pro server in vanilla HL2DM mode.