Pulled from DCEmu.
"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." (Open Sonic)
You can read more about Open Sonic HERE.
There is also an open version of The Great Giana Sister game.
Note that this is based off the original not the sequel that is currently being worked on, Giana's Return.