So, you found all the coins in New Super Mario Brothers? Easy. All the levels on Super Mario World? No problem. If you're after a real challenge, try Hard Relay Mario Brothers. This swirling maelstrom of spikes, jumps and spinning fireballs looks almost impossible to play through. You'll need to take advantage of minor glitches in the game to complete this emulator-only hack. You'll also need patience, skill and balls of steel if you want to make it to the end. Making use of emulator save states might also give you an edge.
A lot of the jumps here involve lining Mario up exactly with the bottom of each block. If this done correctly, the game tries to push Mario through the block, enabling him to jump again. Around the 11.30 mark, more than 20 of these jumps are strung together - impressive stuff.