The Sonic Community is home to very impressive fan works and projects, with rom hacks like Sonic Megamix and amazing fan games like Sonic Before/After the Sequel coming to mind. One of these fan games, is a Sonic Engine called Sonic GDK. It is based around Unreal Engine 3 & offers fan game creators the tool set to make 3D Sonic titles.
Recently, a game set inside this engine released called Green Hill Paradise Act II, which was a project meant to show off how you can make a 3D Sonic title where you EARN your speed, not unlike the Genesis Era of Sonic. Below are two YouTube videos of the game in action from cobanermani456 & RadioactiveRahSNG. Both show completely different paths and routes you can take, presenting how large this fan project is.