Super Mario Crossover
Mixing old and old
This flash game is definitely something for all you NES fans out there: Super Mario Bros. Crossover. Basically, it's the first Super Mario Bros game, except you can play with a selection classic of characters other than Mario; Link, for example, or Megaman. If you haven't tried it yet, better do it before they pull it due to copyright infringement.
On a side note, I'm also enjoying Canabalt now. Make sure to crank up the volume!