From Valve:
From now through October 26th at 10 a.m. Pacific, developers from all over the world are sharing demos of brand new games they haven't even released yet. There are so many demos, we revamped our presentation of the content on the front page to (hopefully) make it easier for players to search by preference of topic, genre, etc.
Join us over on Steam now through October 26th for the October edition of Steam Next Fest: Play as many free demos as your heart desires, and wishlist the ones you want to keep track of to be notified when they officially launch.
