In celebration of PAX East, Natural Selection 2 is free to play on Steam until Sunday. If you like it well enough, you can purchase it at 50 percent off as well. Developer Unknown Worlds expects over 15,000 people to play concurrently.
If you're at PAX East, the Unknown Worlds team can be found on the show floor, presenting a gorgeous update to Natural Selection 2. (That's not an opinion, that's the name of the update.)