You could literally spend hours and hours looking at all the apps that are out there for Flickr.
I found Flickrbits really useful for finding more Flickr mashups. It's pretty impressive. The one that I have found the most interesting (thus far :P) is Retrievr This was really awesome because it uses a sketch that you draw to find pictures on Flickr. So while you are drawing using a pretty basic paint set, say you draw a red dot in the middle of the blank screen it will bring up images of a lady beetle on white space, or a maple leaf on a book or anything which matches the colours that you are entering but if you decide that the image you are actually looking for has purple spots in the corners you can add them straight away and it will be searching while you are drawing. Its very cool when you have a basic idea of the image you are looking for or want an image to match a colour scheme.
It also allows you to search using an image that you have on your computer for other similar images.
It is so interesting and I highly recommend checking it out!