Open Sonic is a free open-source game based on the "Sonic the Hedgehog" universe. It introduces a different style of gameplay called cooperative play, in which it's possible to control 3 characters simultaneously.

Unlike most similar games, Open Sonic provides a greater level of interaction between the player and the levels.

It's more than just a jump'n'run; the user must come up with some strategy in order to get through the levels.

NOTE: You can't create your own levels on Dreamcast version









Open Sonic is available for Windows, Linux, Mac OS X (experimental) and Dreamcast. The latest release is 0.1.2 (July 23rd, 2009). Main features:

• No installation is required + small download
• Sonic-style physics (360° movement engine)
• Available in 4 languages: English, Portuguese, Spanish and French
• 2 levels + innovative items + extras
• You can make your own levels using the embedded level editor
• Good performance even on old computers*

Open Sonic Development Blog

Source

It only took me two minutes two burn this game because its beta. The game play is different then sonic game because you control 3 characters (sonic,knuckel,tails) so you have to work as a team to pass the level.

There are teleports throughout the level and if youre other team gets too behind they will teleport to you, but there are some puzzles that need at least two characters to solve. It sucks that if one character dies then its game over and you have to restart the level and all characters share the same rings.

Because this is beta version there is some slow downs when you get hit and loose your rings and sonic is not as fast as in original games.

More info here