PictureSandbox
Picturesandbox.com uses the Flickr search APIs to return images by all users on Flickr.

FlickrBabel
FlickrBabe borned as a satellite application when exploring several web services APIs trying to understand better how they work to use that knowledge in Wipley development.

Color Hunter
Color Hunter is a place to find and make color palettes created from images. When you search for color palettes by tag, ColorHunter.com searches Flickr.com via Flickr’s API for images labeled with the tag you entered. The thumbnails of the Flickr images are used to create a color palette.

Compfight
Uses Flickr’s API to search the database of photos and then feeds back the results as live clickable thumbnails.

Flickr hack: All Sizes for all pics
The public API gives directions for constructing the url of any photo based on information in the HTML of every photo page.

Phlickr
An open source PHP5 based api kit used to access the Flickr API.

FlickrJ
Java wrapper for the Flickr REST API.

Flickr .NET
A .NET wrapper for the Flickr API. Allows desktop and web applications to provide direct image upload functionality to Flickr and its services.

FlickrClient
Python interface for the Flickr API.

dFlickr
wraps most functions of the Flickr API and provides the necessary classes to access users/photos/photosets/groups/pools/blogs in Flickr.