One of the replayability features in Halo 3 are "skulls", special doodads hidden in obscure places in the various levels for you to find. Finding them is fun in itself and once you've found them you can use them to unlock modes that make the game harder. I can't imagine finding these on my own, so I went online to Halo 3 Planet, Team Xbox, and Hushed Casket to find them. Here's a list of their basic locations, spoilers ahead.
Skull*, Sierra 117 (L1)