Tomorrow, October 16, Valve will kick off their biggest "Free Weekend" event ever. Starting at 10AM (PT), ten games will be available for completely free for you to play through the weekend.

There are literally no limitations on the content (except for DLC) that you can play. You can play single player, multiplayer, or just stare at the game in your list if you so desire. On top of the free play time, you will also be able to purchase the games at a huge discount to keep playing after this weekend.
Beginning at 10 am Pacific Time, 10 popular titles will be available to play free of charge, and available for purchase at tremendous savings.

The 10 titles being offered are:
Blade Symphony
Company of Heroes 2
Don't Starve
Grid 2
Killing Floor
Payday 2
Trine 2
XCOM: Enemy Unknown

All you need to have is a Steam account.