This is an interesting fangame I found a while back; quite possibly one of the best amateur games I've ever seen. Very professionally done; it plays extremely close to the old school Sonic games, with a few interesting differences.
The game follows the plot that Dr. Eggman invents a time machine to return to a point in time where Sonic defeated him, then quickly turns the tables and aids his past self in killing Sonic and Tails. Once he succeeds, he sets into motion a time paradox where Sonic begins fading away in the present. Figuring out what Eggman is up to, Sonic heads for Eggman's fortress to prevent the time paradox from becoming permanent...
http://www.sonicfangameshq.com/view.php?sec=1&id=194