Run n' Roll
After you log in and customize your avatar you can choose from a single player mode or the multiplayer arena. Go solo first to get a feel for the controls, then take your skills out into the wild. You can join players waiting to race by clicking their planet in the sky, or simply start your own game. There are a number of different environments you can choose from to hold matches, each one with a different level of difficulty. The harder the planet, the faster the land scrolls, and the more you'll find yourself falling to the ground.
The entire presentation of Run n' Roll is bright, cheery and friendly. The cartoon-like visual style is quite inviting, and the music has been stuck in my head for days now. Although it's very kid-friendly, users can chat while waiting to play games, so it's a good idea to sit with children who want to play. It's been my experience the players are more interested in running/rolling than chatting, though.
One drawback for Run n' Roll is there is no long-term goal or incentive to keep playing (other than high-scores). Unlockable costumes or other features would be a great addition that might draw players back more often. As it stands, it's an excellent game to pick up and play every once in a while. The skill level of the players varies a lot, so you never know what the next match will be like. Play.
Thanks to mmodule for suggesting the game!