I've stuck mine in a top loading washing machine before.  It doesn't really ruin the rope, however be ready to spend an hour untangling the mess afterwards.  You can get away with washing it in a machine with an agitator if you put it in a pillow case or bag.  Front loading machines are the same way.  You should run the machine through a rinse cycle before you wash your rope so you can be sure there is no residual detergent in the cylinder.  Nikwax makes a good rope wash, I might suggest using an extra rinse cycle also. 
But like Erik says, handwashing in the tub is a great way to do it too.