Indie developer Unknown Worlds Entertainment (UWE) is making Natural Selection 2 free-to-play this weekend through Steam. Natural Selection 2 is a unique, fast-paced, exhilarating game that blends real-time-strategy with traditional first-person-shooter mechanics. Originally a popular modification for Half-Life, the game pits teams of aliens against marines, with each attempting destroying the enemy's base while protecting their own. It was released in October last year to a chorus of positive reviews, and for good reason — it offers some of the most original FPS team combat on the PC right now. The game comes with a steep learning curve, but there are plenty of beginner-friendly servers for new players.

The free offer is timed to coincide with the PAX East games expo, where UWE is showcasing an update to the game that rolled out recently. If you enjoy the free trial, UWE has also cut the price of the full game to $12.50, down 50 percent from its regular price, for the weekend.