This mod is criminally under hyped. It really needs some help.
You play as either the Rebels or the Combine, who of course, try to kill each other. Catch is, when you respawn, it's in the jail of the opposing team (Rebels in Combine jail and vise versa). To get free, one of the still free members of you team has to get into the base of the opposing team and unlock the jail. When all members of a team are in jail, the game is over and you are treated to the death of the losing team, done as the mapper sees fit.
There are only two servers right now, and they are usually empty, but maybe if we can get more ppl interested, that will change.
http://www.jailbreaksource.com/