For something a little more traditional, dive into The Hidden. This mod had a first beta release in 2005 and is still played today, which tells you that it's good stuff. It's a multiplayer team-based shooter with a twist. The majority of the players are I.R.I.S. soldiers hunting down an escaped test subject. And one player is said test subject. As the titular Hidden, you're virtually invisible and can cling to walls and ceilings, but you're mssively outnumbered. It's a sweet take on traditional deathmatch that never seems to get old.

Get The Hidden here.