This weekend's promo at GOG.com includes two of the biggest PC franchises of the '90s: Ultima 1-8 and Wing Commander 1-4 are all reduced by 50% to $2.99 each. I've never played any of the Wing Commander games, so I'm itching to get Wing Commander IV to experience some FMV'd Malcolm McDowell.
Steam has a pair of offers this weekend; Chris has let you know Saints Row: The Third is reduced, so that leaves Worms Reloaded which is free to play this weekend, as well as reduced by 66%. There's a bewildering amount of packs on offer, so my advice is to just stick with the basic game.
On GamersGate, there is a special aliens-themed weekend promo, which can net you big reductions on Aliens vs. Predator, Universe at War: Earth Assault, EDF: Insect Armageddon, Alien Shooter, and Stormrise. Origin doesn't seem to be entering into the whole weekend deals, so there's not much to report from their end. As always, if you see any specific deals, post 'em below in the comments.