I've been using Moo's Mini Cards and I love them. I use them for Business Cards but there is so many creative things you can make with them. I love the fact that you can upload your own Photos and Artwork. Each card can look different. There are lots of Card options too, including Postcards and Stickers.
Here is the Code for 10% off for new Customers:
http://www.moo.com/share/wwwmtmCheers