Steam News announces this is a free weekend for Grey Goo, Petroglyph's RTS game that's also currently on sale. Here's the deal on the deal:

Play Grey Goo for FREE starting now through Sunday at 1PM Pacific Time. You can also pickup Grey Goo at 60% off the regular price!*

FIGHT THE CHAOS. OR HARNESS ITS POWER.

Centuries after Humanity first left Earth, all forms of life in the Milky Way have evolved toward an inward tranquility. But there has been a mutation. A mysterious organism barrels toward the surface of a lone, resource-rich planet known as Ecosystem 9, on a mission to consume all it encounters.

If you already have Steam installed, click here to install or play Grey Goo. If you don't have Steam, you can download it here.

*Offer ends Monday at 10AM Pacific Time