http://www.topeak.com/2007/products/...runkbagdxp.php
Kbrown, this is the one I have...I'm pretty sure. I bought two others and had to return them because they all look kinda similar in the pictures. It definitely had the expandable top and fold out panniers and bottle holder on the back wiht a little tap for attaching a tail light.
In the warmer months, I can fit a change of clothes and my lunch in a gladware and my glasses case easily.
I think all the MTX bags come with velcro in case you don't have the rack to slide it on to.

I also got the rain cover which I'ved used no more than 3 times, more for the visibility than for keeping the water out, that hasn't been a problem.