I suggest Wolfenstein Enemy Territory from Activision, id, and Splash Damage. If you don't have it, be glad cause first, it's a free full game, and second, it's Online Multiplayer Game of the Year 2003.
Read here to find Game of the Year awards: http://www.splashdamage.com
Download the game here: http://www.3dgamers.com/dl/games/wol...olfet.exe.html
Download custom maps here: http://m8d.dyndns.org/m8d/etmain/
There is a Map editor to Enemy Territory (ET), and probably skins too.