Project M: a mod of Super Smash Bros Brawl that adds balance, new stages, new moves and fast-paced Melee style gameplay.
Also supports online play for the new tracks. There aren't a whole lot of extensive mods out there. They're not as extensive because they were kind of the prequels to Newer NSMB but they're all still of great quality. Brawl - There's a few different hacks on here, mostly just tweaks to gameplay.
Haven't played it yet so do take this with a grain of salt. Hopefully the quality of the maps improves, some levels look pretty unsightly. On first look i'm not too impressed but it doesn't look bad either, probably worth the download. It's not alot and most of the stuff is short but here it goes: New Super Mario Bros. I'll also post the one's given to me in the replies of the thread. I've searched in vain over the internet for a list of some of the best hacks out there, so i was wondering if some of the posters on here could either point me to some list's or tell me of some of the better ones they've played.