If you haven't given Arkane Studios' Dishonored a shot then you've got a great chance to do so this weekend as it is available free on Steam. As you'd expect there is a 75% discount to outright buy the game during the promotion if it catches your fancy. Gearbox Software's Borderlands 2 is also free this weekend on Steam.
Speaking of Dishonored it's currently the free game that's offered via the Games with Gold promotion on Xbox 360 - and that's a copy of the game you get to hang on to, so make sure you download it if you've got Xbox Live Gold.