Sonic the Hedgehog 3D is a free fan game about a famous SEGA mascot, Sonic, and it is made on Build Engine with additional EDuke 32 port. The game was inspired by two classic Sonic series: Mega Drive/Genesis and Sonic Adventure series.
The free game is a perfect 10. It certainly plays like Sonic, and I love the old Sonic and this version.
What is not perfect is the engine, the graphics do not need to swallow up all those resources, but this is what it is and not part of the equation.