I've been promising myself a set of properly prepared MOO cards since I got some free samples a year ago.
I finally got round to doing this; the cards arrived just in time for my trip to the Internet2 Fall Member Meeting, and literally an hour before the postal strike started.
For this batch, I went through a lot of old images looking for images that would work in the 70mm by 28mm format; most of the results are still crops from much larger images, though. One thing I found very helpful this time round is that you can download a Photoshop template for the card format, including guides to help you allow for a safe area and off-card bleed.
I've uploaded all the images I used for this batch as a flickr set for anyone who is interested; they are CC-licensed, so feel free to use them for your own cards if you like them.