GoldenEye Source

This is a game that has been around for a long time. It is constantly getting updates though and becoming better and better with each one. Basically, it's like a fan made updated version of the one and only, Goldeneye 64. Except it's a half life 2 mod. Although it has no single player campaigns, it has legendary online multi-player.
It's probably better just to give you some links, you can look at videos and such at the site. Well worth a look.
http://www.moddb.com/mods/goldeneye-source