I drop them in restaurant fishbowls and have scored a few lunches. There used to be a shop where I needed proof of having a business. I developed them originally for that shop.
I do same as above, also use them for introdution as they have my cell and email address. They have no specific information, just a business card, saying I am an Independent Contractor.
Vista Print has cheap business cards. Now they don't even have their logo on the back. I've been really happy with them. Their online card designer is slick, too.
And of course there are the print them yourself business cards that come with micro-perf. Once again I use Excel so I can adjust cell size to accommodate logos, print sizes and fonts :-)