Timbuk2 makes some. I have this one...
http://www.timbuk2.com/tb2/products/...ckpack/2114822
It holds a good amount of stuff and is pretty comfortable. I do plan on making some modifications this year though. I am going to add a waist strap for when I have really heavy loads and also a compression strap around the top. The top squared-off corners make it harder to see behind you while riding, so the compression strap will pull those corners in and out of the way.