This unofficial project ports back most of Sonic Unleashed's day levels from the Xbox 360 version into Sonic Generations on PC. It also includes any improvements that are deemed as necessary to accomodate to the better control scheme, higher resolution graphics, and a much smoother framerate.
Now this, here, is a labor of love. Salvaging the good parts from Unleashed and bringing them to PC, polished up and improved as a sizeable Generations expansion. Gotta love it.
Here's hoping Sega don't drop a cease-and-desist on it. They love to do that.