Super Mario Goodness

If you belong to the subset of people who still consider SMB3 one of the finest pieces of video game ever made, you might want to check out Mario Adventure. This "hack" is in reality a complete game unto itself, with new worlds, new levels, new monsters, but based on the SMB3 engine. It's fun, insanely difficult, and 100% Mario. You can download the rom here. All you need is a NES emulator and you're set.