I don't give a briss if this is old, I'm not as up to date as you nerds.
I fucking hate rom hacks, I have only played one for more then one minute (The Mario Streetfighter one) before turning it off. But this rom hack of Super Mario 3 that I recently come across is above and beyond fantastic. New Levels, New Enemies, New Mario Types, New Weather System, a completely brand New Game almost.
In fact, it may be the best game I have ever played if it wasn't so damn hard. You may want to cheat somehow (save states, but I wouldn't use game genie cheats only because you'll miss out on the new mario types)
Read this review of it which has a download link at the end, or not, see if I fucking care.
http://www.vintagecomputing.com/index.php/archives/63