The Battle for Wesnoth is probably one of the all time great open source games. It's been around for a long time, it's well polished and now it's looking to get on Steam.
Help it by voting for it on Steam Greenlight here.
If you have never tried it or heard of it before, you're seriously missing out on a lot of fun:
- Over two hundred unit types, sixteen races, seven major factions, and hundreds of years of history
- Sixteen exciting, full-length campaigns
- Multiplayer and co-op (LAN or hotseat) support
- Support for creating your own exciting scenarios, maps and campaigns
- A large, dedicated community of fans and modders