Forgot one Flickr stats tool: Group Trackr. It tracks membership numbers and pictures posted over time. It doesn’t need the massive amount of requests that would be needed to do the same for a user’s photostream. The results can be posted as a static image on web sites etc.
Here is the Processing Flickr group:
Group Trackr has introduced many new features since March, including detailed, group analysis, full activity reports and comparative graphs, check it out!