Two very different games, Gratuitous Space Battles and the multiplayer component of Call of Duty: Black Ops II, are freely playable this weekend on Steam. Both titles are also discounted -- down to $3.74 and $40.19, respectively -- should you want to permanently add them to your library.
If you're just in it for the free entertainment, however, know that both will be free to play until 1:00pm on Sunday. You don't need me to talk about Call of Duty, but I will say that GSB is worth the time it takes to download. Fast-forwarding through space wars shouldn't be this much fun.