rFlickr
Warning: This gem only works with Ruby 1.8, a new version compatible with Ruby 2.0 is coming soon.
To use this version of rFlickr, first you will need to install the RubyGem using the following command:
$ sudo gem install digitalpardoe-rflickr
Then you'll need to get a Flickr API key as detailed here:
http://www.flickr.com/services/api/misc.api_keys.html
You'll need to authorize your application's API key from each Flickr user account which wants to use it.
A short guide to get you started can be found here:
http://digitalpardoe.co.uk/blog/show/87
The guide is a little out of date in it's gem installation technique, use the instructions above instead.