I used a glitch called a "Credits Warp" to skip straight from the first level to the credits. By doing so I beat the game (at least by some definitions) faster than any human ever has before, in 4:49.8.
Credits Warp Info: http://deanyd.net/smw/index.php?title=Credits_Warp_Run
Credits Warp Info: http://deanyd.net/smw/index.php?title=Credits_Warp_Run