Well, I just love Flickr! I have been using their photo storage service for over a year now to back up my felting photos as well as some of my personal family photos. I have found it to be a great place to network with a large community of people who have all kinds of interests. There is a group there for just about everything, and if you don't find what you want in a group, then you can make your own. Below are a list of a few direct sites and article posts that offer services and tools to go along with your Flickr account. I have found these to be very helpful in using my Flickr photos to the fullest. I hope that you find them to be useful too.
Big Huge Labs has lots of cool tools! I used their mosaic tool to create the image above.
Slide allows you to make neat slide shows and more with your Flickr photos and other apps too.
Quick Online Tips has an article with a huge list of sites related to Flickr toys, tools, and more.
PedagoNet also has a big list of sites for Flickr tools.
FlickrTools is another list of sites for more toys and tools.