Grey goo is free to try this weekend, but then, when is it not? Simply go out into the street and taste that squishy thing on the pavement—or, better yet, don't ever do that. Wait. Please stop. However, Grey Goo the game is only free to try this weekend, because, you see, Petroglyph's RTS is having a free weekend on Steam (opens in new tab).
You know how Steam's free weekends work by now, I'm sure. Valve automatically, and slightly annoyingly, adds said game/s to your library, and then you look at the size of the download and never install it. Or, if you're not me, maybe you give it a go.
It might be worth it this time, as Ian Birnbaum rather liked the old-fashioned RTS when he reviewed it back in January (opens in new tab). Grey Goo has since been expanded with the 'Emergence' campaign (opens in new tab) which, like the main game, is currently 60% cheaper for the next few days.