This is an amazing video I came across – this player beats Mario 64 in under 17 minutes with only 16 stars. There are some glitches exploited, but its still pretty unbelievable.
You can videos like this for tons of old games on this website: http://bisqwit.iki.fi/nesvideos/. I watched a few – especially Marble Madness, Super Mario Brothers 3 (which is amazing), and Excite Bike. Pretty neat, but also goes to show people have too much time on their hands!