Vistaprint - does FREE printing on a lot of items -including business cards- as long as you are willing to have the Vistaprint logo on the back.
Overnight Prints - my favorite online printing source. The cards I used for the GAEA conference were printed from this source. I always use the economy printing, and it never looks economy.
Zazzle - they print stickers and other promotional stuff like that. It isn't cheap; but it is awesome.
Shutterfly - they advertise as a source for printing professional-looking photo albums and scrapbooks but they are great for printing portfolios etc. etc. Average cost is about $30 for a hard-bound book.
Lulu - another online book printing source. They have a cheaper option if you plan to print out duplicates of anything.