How can I add an amp to a receiver w/o amp outputs

I've heard that there is a device to take the standard output from an old receiver and allow you to add an amp to that output. What is this piece called? What are the good ones?
There is a device that allows you to use the speaker outs to connect an amp. It coverts the speaker outputs to RCA. You can get them at Rat Shack I believe.
You could add a Creek OBH-10 passive remote volume control. Use the "tape out" on your receiver.
Why would you want to? Why not buy another receiver or preamp?