If you're looking for a good way to wind down the work week, you'll want to pick up Asphalt 8: Airborne on the App Store since it's ditched the $0.99 pricetag and gone free for this weekend only. There are a few freemium elements in the game, but if my glowing review is any indication, they sure don't get in the way of anything. The graphics are fantastic, soundtrack thumping, and with tons of customization and multiplayer options, you'll have a hard time getting anything productive done this weekend.
Asphalt 8 not your cup of tea? Real Racing 3 is the next best racer on the list, especially if you're looking for something less arcadey and more realistic. For something more casual, I've always been a big fan of Forever Drive.