Adding spikes to the bottom of your speaker cabinets can be a very economical way to improve their sound by isolating them from the surface they are resting on. In my case, the floor of my apartment. This modification was done primarily for the benefit of my downstairs neighbors, because I like to play bass heavy music and getting the speakers up on spikes will reduce the amount of bass transmitted through the floor.
That's the idea, anyways.
Careful if you just have hardwood floors. If you support heavy speakers this way, they will dig right into the wood. You can be creative with protecting hardwood floors. One common method is to put pennies under the spikes. It is also common to see spike sets sold with small platforms for this purpose.
Here is a lengthy article on what may or may not happen when using speaker spikes.
noahw also has a short write up on this subject on his instructable.
Step 1: Parts List
Speaker spike sets, I got the cheapest ones I could find since I didn't pay anything for the speakers and this isn't audiophile gear.
You can get pretty fancy ones if your gear warrants.
Hammer or Allen Wrench depending on the design
Adhesive - A couple reviews at Parts Express recommended using an adhesive like Liquid Nails to improve the seal of the threaded inserts. I picked up something called Seal-All at the hardware store, but any decent glue should do the trick.
Vacuum Cleaner to suck up saw dust (for the carpeted apartment hobbyists out there).