From now until February 28th at 10am PT, you can play as many demos from upcoming games on Steam as you like for FREE! (Just don't forget to wishlist your favorites.)
There are also lots of chances to interact with developers, from live developer chats to a whole week of livestreams. Take a look at the schedule on the front page to plan out your week, and check out the official event livestream today from 11am - 1pm PT.
Have fun!
-The Steam Team
*More than a few; less than a gazillion.
Here's some of the demos available