Starting tomorrow, Serious Sam HD: The Second Encounter is getting a free weekend of multiplayer on Steam. From then through Sunday, you'll be able to play co-op Survival and Campaign, deathmatch, and other modes in which you shoot dudes in the face.
I couldn't not include a snippet from the press release:
Lastly, for the duration of the promo, The Second Encounter will be marked down to $10. That's half price!