Moo.com is one of a number of on-demand printers that provide some useful services through a web-based interface.
So far, I've had two sets of business cards and two sets of mini-cards from Moo and I'm very impressed with the quality and finish of the printwork, the price, the packaging and how easy the process of making the cards was.
Moo.com provides a web-based solution for setting up your cards. For mine, I chose 30-odd images from one of my flickr albums and used these as the fronts of my cards, setting up the backs with Moos tools. All very easy, and the results look great! At £10 for 100 mini-cards I recommend trying it out!