I have a Wald paperboy rack. Boy, can I stuff that thing! I could fit 2 gallons of milk on each side, and then more stuff on top. Then pizzas on the flat part. The best part is that it will slide right over a Blackburn Mountain rack, and slide right off again. Sorry, no pics handy...
What I usually carry depends on where I'm going. I have a saddlebag/briefcase for work, and a backpack/messenger bag for school, probably 5-10 lbs. worth of what-have-you in either at any given time, plus the usual array of bike tools in the seat bag.