Right now you can download and play the full version of Serious Sam HD: The Second Encounter from Steam for free. The promotion runs until 1PM Pacific tomorrow, that's July 11th. During that time you can purchase the full version of the game for 50% off, $10.
Serious Sam HD: TSE comes with full 16 player co-op support through the campaign as well as a survival mode to play. It's also the first SS game to come with versus multiplayer. All of that is wrapped up in a shiny and gorgeous looking package of new graphics. You can check our review for more.
Source: Store.steampowered.com