I don't know if this is dangerous so if it is please remove it Wozzaaah but i have heard of a few people using the smaller ones as a tank for a small airbrushing compressor,infact i have also seen a how to guide about it in a book published by kestrel books
There's a bloke over th'ill in Yorkshire called Mick Kirkby Geddes. He uses all sorts of things like fire extinguishers to make sculptures. Don't know if you were looking for more practical applications though? Here's a picture of one of his pieces.
[ i made one out of a fire extinguisher / there not a bad size and hold a fair bit of media and well portable . i allso made one up for damp proofing . bought the probes and made the pipes up brazed the fittings on , filled it up and presurised to 90 psi of comp