Here is one great use for WD-40 for you mountain bikers out there who ride in the muddy slop:

Spray the WD-40 on the down tube of your bicycle to keep the mud and guk from sticking (permanently) to your frame. Washing the bicycle off later on will be an easier job. This is also useful to keep the salt (in the winter time) off your frame. (But please, don't spray it everywhere if you don't want the smell.)