ZOMG HAX!!! Seriously though, it seems like Super Mario has a buttload of hacks out there, some of them really well-made and even worthy of putting on a repro cart. So, how is Sonic holding up? There are a few decent sites dedicated to hacking Sonic games, but after going through them, I found it's kinda hard to pick out a good list of well-made, completed hacks. There is a lot of ugly unplable rubbish out there, and a lot of projects that seemed like a good idea at some point in 2004 but have been abandoned for years. However if you look around hard enough you can find some gems.
Here are the best ones I found:
First off you have the essential Sonic Megamix, probably the most intense hack out there (even though it's apparently not done yet) this hack is pretty awesome. It runs on Sega CD (or emulators) and features added characters Mighty, Shadow, Knuckles and Tails, and apparently you can activate their super forms, though I never could get it to work... even with all the gems...
Next, Sonic 2 Retro Remix is another awesome hack with a really well done adventure mode that has you exploring the huge levels looking for for specific objectives. Very original and well made. This is probably my favourite hack, it's also the most complete one I could find.
Sonic 3 Complete has to be the most awesome idea for a Sonic hack I've seen to date, the fact that no one did this before is kinda amazing. It's Sonic 3 + Sonic & Knuckles, together as one game, FINALLY! Other than putting the 2 games together as god always intended, this hack fixes a bunch of glitches in the original games. The only issue I have with it is I can't seem to get the super and hyper forms to work. Apparently you have to hit B and C when double jumping to initiate them, but it never works, either that or the all emeralds codes don't work. Either way, this hack is still pretty awesome, and hopefully they'll make an update where the super forms work better (or at all). I'd love to get this put on a cart.
The S Factor: Sonia and Silver is another very well done hack featuring characters not seen in other Sonic hacks, apparently this thing isn't finished yet either, but regardless it plays like a dream. The level designs are very expertly done, the music is very nice and the characters have really cool special moves. I'm just not sure about the inclusion of Scourge The Hedgehog... but regardless this hack is pretty sweet.
Sonic Boom is a mindblowingly AWESUM hack that features new: essentially everything: sprites, enemies, level backgrounds, tiles, moves and attacks, boss... basically everything. It's ridiculously awesome. The only problem is... it's just one level and the boss. That having been said it's probably the best 5 minutes you'll spend playing a Sonic game since 1994. Oh how I wish they'd made a full game of this.
There are a few other honourable mentions out there like the Knuckles in Sonic 1 hack, but that's been around forever and I'm sure everyone has seen it already (if not, google it). Plus there are a few hacks that restore the lost levels to Sonic 2 like the desert and forest levels, but they seem to suffer from somewhat clunky level design and don't really bring that many new features other than the extra levels.
Those are the best ones I could find. If anyone on here knows of some other sweet hacks please share!