Battle for Wesnoth, one of the oldest and most polished open source games around is now available free on Steam.
The developers aren't just celebrating finding a new audience on Steam, they also just pushed out a big update to the game as well, what they're calling the "New Horizons Update".
They even made a shiny new trailer for the release:
With this new release, they're now requiring you have a registered account on their official forum to play online. They say this is needed as a result of "issues with abuse of unregistered nicknames". They need to fix a tooltip that says it's optional though, which will be in the first point release 1.14.1.