Geoff Mendicino - http://twitter.com/mendicino (Me! 360, PSN giveaways; also sometimes alert others via twitter of other contests)
FreezeCracker - http://twitter.com/freezecracker (My website, gives away codes sometimes and alerts to our website's contests)
Major Nelson - http://twitter.com/majornelson (Official Pimp of Microsoft, gives away XBLA nearly daily, but has a lot of followers)
AceyBongos - http://twitter.com/aceybongos (Cool Microsoft guy, less followers but gives away a lot of XBLA)
Official Xbox Magazine - http://twitter.com/oxm (Doesn't give away as much as before, but they'll give away the occasional XBLA)
dennya - http://twitter.com/dennya (Another cool Microsoft guy giving away cool MS codes)
TellTaleGames - http://twitter.com/telltalegames (360, PC giftcode giveawayers-- Very often!)
Trixie360 - http://twitter.com/trixie360 (She's caring for what Major Nelson calls her "spawn" now, but she has had huge giveaways)
Sega - http://twitter.com/Sega (Sega games for 360 and PSN! Also free swag as well.)
RunningPixel - http://twitter.com/RunningPixel (Italian developers who give away free copies of their PC games)
nuka_cola - http://twitter.com/nuka_cola (Fallout 3 DLC giveaway codes!)
Let me know if there is anyone else who should be put on this list-- also let me know what they usually give away.
Edited by Geoff Mendicino, 07 August 2009 - 11:59 PM.