Halo 2. A jumpy little number.

The first amusing exploit I remember in an FPS was rocket jumping in Quake. Always fun trying to perfect that, and splattering yourself all over the map while you learned to do it.

There were jump exploits in the original Halo apparently, but someone has discovered a rather amusing tank jump exploit in Halo 2. I've not played the game (sadly), so have no idea if this has any practical use, but it sure makes for very amusing viewing.

Since the site has been getting hammered, here is a bit torrent link in case the site goes down. If you need a Bit Torrent client,
Azureus is a good one in my experience.