Picniche [3] is a stock photo supply and demand analysis tool. It analyses the number of sales generated (demand) for a given keyword/s against the number of hits generated (supply) on several microstock [4] sites.
Saturated photos (lots of supply compared to demand) generate a small rating, less well covered topics (more sales vs the number of available images) generate a larger number. Try searching for 'apple' and you will see that while there are lots of sales there is also lots more competition in the form of images available. The top/recent page contains some interesting high scores so if you are stuck for photo ideas here is one place you might find subjects in demand or subjects where your images will have to stand up to very little competition.
The site also suggests keywords common to images found serving as a keyword suggestion tool.
A handy history at the bottom of the page shows your most recent searches.