A team of Sonic fans is currently working on an open-source fan Sonic game, called Open Sonic. According to its description, Open Sonic introduces a different style of gameplay called cooperative play. As such, players can control 3 characters simultaneously. Moreover, and unlike most similar games, Open Sonic provides a greater level of interaction between the player and the levels.
This basically means that players must come up with some strategy in order to get through the levels. In short, this is more than just a jump’n’run game.
Open Sonic’s latest version is 0.1.4 and is available for download. This version features one level (Sunshine Paradise) and nice story with charismatic characters. All proper physics have been implemented (slopes, loops, etc) and it comes with multiple language support. Furthermore, it features a level editor and a moddable engine, offers great performance even on old computers, and the complete source code is available to everyone.
Those interested can download this free fan game from here.