You're looking for PC parkour games to buy on Steam, but what the heck is a parkour game anyways? You might call these PC running games, since some of them have you constantly running and reacting to what's in front of you. These PC games all have you doing a lot of climbing as well, as seen in the Assassin's Creed series, and games like Dishonred 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 still is the truest parkour-like game out there, so definitely check that out if you missed it before.
Vote up your favorite PC parkour games on this list, and downvote any titles you played but didn't enjoy as much.