There are a few products that will do this. Basically, you buy a box that goes in between the speaker output and the speaker itself. I just did some quick Googling and found these products. Also, Hot Plates have a line out. In case you don't know, a Hot Plate is a power attenuator manufactured by THD. This allows you to get the TONE of your amp cranked at a reasonable VOLUME. Just make sure you get the correct impedance for your amp!
Any of these will serve you well:
http://www.suhrguitars.com/tonetools.aspx
http://www.hughes-and-kettner.com/products.php?mode=prod&id=110
http://www.radialeng.com/re-jdx.htm
http://www.thdelectronics.com/product_page_hotplate.html