It's usually difficult to find high-quality free racing games, but sometimes you do find exceptions to this rule like Moto Race Challenge. This game doesn't offer the official championship bikes or pilots, but at least you can play it absolutely for free.
Graphics are better than we expected, with good response to controls. The motorbike is controlled just with the cursor keys, but don't let this apparent simplicity fool you: it takes some time and practice to master it, especially when accelerating and braking.
You can play offline as much as you want, but if you'd like to join an official race or championship, then you must register as user (it's free). However, bear in mind that you'll have to pass the qualifying series to be allowed to join the race.