Cyclo Maniacs
Ride your racing bike with WASD or arrow keys. X or spacebar to perform a bunny hop, B to use your horn.
http://www.gamesfreak.net/games/Cyclo-Maniacs_3730.html

Dirtbike Fun Flash Game
Have fun riding your dirt bike through challenging obstacle courses. Try to reach the end of each course without crashing too many times. http://www.playmotorbikegames.com/games/Dirtbike_Fun.html

Dirt Bike 4
The goal is to complete all levels in the shortest amount of time without falling off your bike. The quicker you finish, the higher the score. Try to avoid odd angles when jumping off the rooftops and try to get the bike stable at all times.
http://www.tripledoublegames.com/dirtbike4.html

Mountain Bike 2
Perform some insane stunts and ride over some rough terrain in Mountain Bike! Watch your landings and take care on the hills, or you will fly off the bike and roll down the mountains.
http://www.tripledoublegames.com/mountainbike2.html