You should already have seen that Black Ops II is free to play this weekend and 33% off to own, and the Bit.Trip games are reduced all week so it just leaves me to tell you that Trials Evolution: Gold Edition is 25% off this weekend. Over on Amazon you can get Hitman: Absolution for just $4.99 and GTA IV Complete for $14.99.
GOG.com has got a great promo this weekend, reducing some recently released adventure games like Resonance, Gemini Rue, To The Moon, and Incredipede to as little as $3.99. GreenManGaming has special daily deals on the Total War franchise this weekend, right now you can get Total War: Shogun 2 for just £6.24, which is 75% off it's regular price.
GamersGate has a big selection of Bethesda titles reduced to half price this weekend, including RAGE, Dishonored, and Fallout 3. That's your whack for now, if you find a good PC and Mac weekend deal, leave it in the comments.