Here's an old trick in Super Mario Bros that allows you to get unlimited lives (well, really just 99, but who's counting?! :). This "glitch" in SMB is on level 3-1.. you'll see two turtles walking down the step blocks, stay on the 1st step and jump on the turtle as it drops onto the 2nd step. Keep jumping from the 1st step and watch your 1UPs rack up! If you're not sure how to do this, watch the video below.

There's also a way to do it on the New Super Mario Bros for Nintendo DS. Here's how: