Three games featuring Sonic that can be played online for free are "Ultimate Flash Sonic," "Sonic Smash Brothers" and "Sonic Racing Zone." These games and many more can be found on AllSonicGames.net.
A side scrolling adventure game, "Ultimate Flash Sonic" is a remake of the Sega game "Sonic the Hedgehog 2" in flash. Sonic, Tails, Knuckles, or Cream must navigate through each level as fast as possible while picking up the classic gold rings.
"Sonic Smash Brothers" pits characters from the Sonic game world against one another in a free-for-all battle. Each character uses attacks and falling items to knock the other fighters off of the stage, each fall costing the fighter one life. When a character has no more lives he loses, the last one on the stage wins.
"Sonic Racing Zone" is a side scrolling racing game featuring a cast of Sonic game world characters. This game is more than just crossing the finish line before the other racers. Each stage has the sloping hills and intense jumps iconic of the original sonic games and a physics engine that requires the player to stabilize his racer's car between each jump. The result is a simple but challenging racing game done in the style of the old Sonic the Hedgehog games.