This one's good for us younger guys who can't get a hold of all the sorts of toys and contraptions and things that people talk about on here.
This will only work if you're the right height, but if you ARE the right height, it's just awesome. Kneel on the ground in front of a toilet in your bathroom (I usually put a mat or a towel under my knees). Now, with some sort of protection against chafing (I use bundles of toilet paper. Improvise with what you've got), stick your dick on the edge of the toilet bowl so it's hanging over the water below, then pull the seat down over it. Pump away. You can press down on the seat or ease off to control pressure on your dick.
Closing my eyes and imagining I'm fucking some chick doggystyle gets me off so fast. Cleanup is great, too. Everything goes right into the toilet; just drop the TP (or whatever you used on your dick) in and flush away.