At present, the API is in beta - we think it's mostly done, but we might make a few changes before we 'go 1.0'. We expect any changes to be backward-compatible at this point. What's more, when we go 1.0, we'll be sharing revenue back with the API key holders (and we'll backdate for any packs made during the beta period). The exact revenue structure is still being worked on, and we'll announce it nearer the time.
At moo.com/api, you can sign up to play, find the documentation, and join the developer's mailing list.
So, what can I do with it?
Simply, if you have some images that you want people to be able to print, you can build an integration to connect up to MOO.
I'm more excited by things that people will make that we haven't thought of, but here's some things that have been done:
I hooked up the TheyWorkForYou.com API to our API to produce a stats-based political trading card game, a project I've been meaning to do for about 5 years now.
Dan Coulter (of phpFlickr fame) has written a PHP wrapper library that makes building MOO API applications really very simple indeed, and a couple of sample apps
Basic - add some images, make a pack. Basic, indeed.
Mosaic - takes a big picture and makes pack of minicard tiles from it.
Rainbow - makes a pack out of your most interesting flickr photos, sorted by colour.
Alberto from Tagmore has a 3D barcode sticker generator.
I'm looking forward to many more!