I like the Super Tourist DX line by Topeak: http://topeak.com/products/Racks/Sup...tDXTubularRack.

It can be had with or without a spring, or there's a disc-compatible version (which I have) as well. Works very nicely for a trunkbag/pannier combo as the lower sidebar keeps the pannier mounting points out of the way. I paid ~$40 for mine.