You're looking for parkour games to buy on Steam, but what the heck is a parkour game anyway? You might call these PC running games since some of them have you constantly running and reacting to what's in front of you. The best parkour games all have you doing a lot of climbing as well, as seen in the Assassin's Creed series, and games like Dishonored as well.
It's hard to argue that Mirror's Edge isn't the ultimate PC parkour game on Steam. Although the game is almost a decade old, it's still one of the truest parkour-like games out there, so definitely check that out if you missed it before.
Vote up all your favorite parkour video games on Steam, and downvote any titles you played but didn't enjoy as much.