http://store.steampowered.com/sale/free_weekend_weekend

This weekend, play 10 different games for free during the weekend!
starting now until 4 PM EST (1 PM PST) on October 19, 10 games will be available to play for free when you log into your Steam client. These games are only free until October 19, after which, you can keep playing by purchasing the game. If you are interested in purchasing these games, you're in luck as they are also on sale for up to 80% off this weekend.
happy gaming~