I saw a live experimental music show once where an electric guitarist put a metal plate on his strings by the pickups and then proceeded to lay a small metal chain on the plate. when he strummed the strings, the result was extremely cool and extremely distorted "metal" -- pardon the pun.

i'll post more of this show once i remember it.....

After stringing up your guitar, leave the excess guitar string hang from the headstock and insert the ends into a high spinning fan blade.

Copper plectrums can be somewhat interesting to fool with as well.
dB-

I once played on the same bill as a guy who did a solo set which involved laying his guitar flat on the stage and making sounds with it using the vibrations, air-flow & magnetic fields generated by a variety of appliances, which included several different electric razors, hairdryers, fan heaters and pocket fans. It sounded fucking amazing, the whole venue was spellbound. I happened to see him play again a couple of weeks later, and was amazed to see him replicate the same set. He wasn't just fucking about, it was obvious that he'd spent a lot of time over it. Pure gold.

Would that happen to be Keith Rowe?

If memory serves, his name was John Cromar

holding the guitar flat and rolling marbles up and down the strings with a delay pedal is kind of fun.

you can get a bunch of weird efects using the electro harmonix polychorus pedal. www.ehx.com

Scream into the pickups. Works especially well with single-coil pickups and heavily distorted guitar sounds.

The band Bigwig used a drill on their last album to an interesting effect. Basically they just put the drill bit on the strings and vibrated them with the rotation of the bit.

Fred Frith (in his Skeleton Crew days with the late Tom Cora) used to build these little guitars out of planks of wood and drill into them. Bit of a different sound there, I'd bet.

If you mail your guitar to Alaska and put a lot of chorus on it, you can get some pretty cool sounds.