I've been playing a little Alien Swarm this week. It's a pretty good game as is, but that's not why it was provided free of charge. No, it was provided free of charge so you kids could mess with it.
And we're already seeing the fruits of these labours. Shrimpsess over on the Steam forums has posted a long list if command line tweaks you can make to the game, which with just a few characters can introduce a range of features other developers would be charging for as premium downloadable content.
Like the introduction of a "director mode", like the one found in Left 4 Dead, that intelligently spawns enemies. Or an actual, working first-person mode for the game. There are also some minor (though welcome) changes, like shifting the Gears-of-War-a-like "fast reload" system underneath your guy instead of hiding it in the corner and enabling 360 controller support.