Halo for the Atari 2600 (but you don't need an Atari 2600)
This requires astounding amount of geekery shooting off in many directions. A gentleman named Ed Fries has built a version of the incredibly popular first person shooter game Halo for the Atari 2600. You know, the Atari that was popular in the early 80s. It's available in browser form and even has 8-bit theme music.