Flickr.rb is an 'insanely easy' Ruby library to interface with the world's most popular photo-sharing service, Flickr.

Here's some example code:

require 'flickr'

# basics
flickr = Flickr.new                           # create a flickr client
flickr.login(email, password)                 # log in for actions that require it
flickr.users                                  # get all users currently online
flickr.photos                                 # get the 100 most recent public photos
flickr.tag('red')                             # search for photos tagged with 'red'
flickr.groups                                 # get all active public groups

# working with users
user = flickr.users('sco')                    # lookup a user by username
user = flickr.users('sco@scottraymond.net')   # or email
user.name                                     # get the user's real name
user.location                                 # and location
user.photos                                   # grab their collection of Photo objects...
user.tag('red')                               # search their photos for the tag 'red'
user.groups                                   # ...the groups they're in...
user.contacts                                 # ...their contacts...
user.favorites                                # ...favorite photos...
user.photosets                                # ...their photo sets...
user.tags                                     # ...and their tags

It gets way better than that though. If you're into photos or just cool APIs, check it out.