SuperTuxKart - 3D arcade-style MK-like kart racing game
Alto's Adventure - An endless snowboarding odyssey
TrackMania - The most popular online PC racing game.
Hill Climb Racing 2 - Sequel to the most addictive and entertaining physics based driving game ever! And it's free!
Speed Dreams - Speed Dreams is a 3d cross-platform, open source motorsport simulation and racing game.
Happy Wheels - The fun Happy Wheels game features several levels that you have to get through using a variety of wheeled vehicles. There are obstacles that are quite challenging. Read more about Happy Wheels.