There must be thousands of ROM hacks out there these days. Most are dross. A few are quite good. Unfortunately it's hard to know which is which without being a member of a community devoted to the game in question...and that community is necessarily going to be smaller than the total number of people who play that game. So I thought, why not have a thread for posting hack recommendations?
Unfortunately, the only game-specific community I'm at all connected to is the Super Metroid community, and good hacks of that game tend to get submitted here. But I'll post the stand-outs anyway:
Super Zero Mission: nominally Metroid Zero Mission ported to the Super Metroid engine, but it's really a sprawling Super Metroid hack that uses the MZM terrain sprites and takes inspiration from some of its mechanics (like the "unknown item" upgrades). Harder than the original Super Metroid, but not gratuitously so.
Super Metroid Eris. A really beautiful exploration-oriented hack which really shows off what can be done with the engine graphically-speaking. Lots of very unique environments, which really helps the exploration since it's easier to remember what is connected to what. Some items are rather well-hidden, though, including the morphball (which is
hidden under a bridge). Again, harder than Super Metroid but not gratuitously so.
Super Metroid Redesign. Even though I personally don't like this hack all that much, you have to recognize its groundbreaking nature -- it was the first SM hack, to my knowledge, to add new mechanics instead of just rearranging terrain. The physics is tweaked, there's a couple of new upgrades, you can shinespark into slopes to start running again, etc. This one is pretty danged hard, including a hell run (going through hot rooms without the varia suit) that has a small margin of error. The altered physics will also probably trip you up.
So, what are your favorite hacks for your favorite games? :)