ROM hacks may be considered as slightly shady by some sectors of the gaming community but they lead to some truly amazing games which - under normal circumstances - simply wouldn't exist otherwise.
Take Sonic 2, for instance. Regarded as one of the hedgehog's finest adventures, it's hard to see how it could possibly be improved - until you add Yoshi from Super Mario World 2: Yoshi's Island, and then it all becomes clear. Sonic + Yoshi = awesome.
This Sonic 2 hack was made by Xenowhirl for the Sonic Hacking Contest 2010, but has only recently surfaced online. If you've played it then let us know by posting a comment below.