If you're in to Source mods, you may remember Insurgency as one of the most popular SteamWorks supported mods on Steam from yesteryear. The mod was a fast-paced tactical shooter which reached over 1 million downloads as well as winning mod of the year.
Insurgency 2 is it's sequel but it will be a commercial game developed by the same creators, New World Interactive. The game has been in development for a year already (still on the Source engine), but the team need a financial injection to push the game quality up a notch.
You can check out the kickstarter here or the Insurgency 2 website.