These guys made the script that I used to make card images appear on this site (just surround the card name with an a tag with class 'mtgcard'). They also make WordPress plugins.
This interfaces with the Gatherer data, which includes all textual information about a card and rulings.
This is the C++ code that runs the Cockatrice client, an app for making decks and playing games.
Official Ruby library
Offers lots of images as well as card info