For those of you that are familiar with open source programs this will be of interest
For those that arnt this could be of interest

But first for those that dont know what it is heres a short introduction
A "open source" program is a freely programmable tool/game
Well known ones are Mugen, Beats of rage, Frets on fire to name but a few

But heres one based on Sonic
Link to the site
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.
Now its not quite finished yet
But hay its better than those recent 3D Sonic games from the sounds of it