The Flickr API rocks. It helps that the developers are really excited about web services (PDFs converted from their original PPTs).
Anyway, there are code libraries available for PHP4, JavaScript and others. Michael Madrid’s Oberkampf is a dead simple PHP library that looks easy enough for non-coders to use. And I found myself quite satisfied with the REST request format and the XML to array parser by Eric Rosebrock.
I used the API to help me tag a bunch of photos with geolocation information. More on all that later.