I have always been interested in Linux distributions and how they evolve, not just technically, but also in terms of their popularity and their -wanted or unwanted- position among the Linux users and on the broader market.
In this regard, the Distrowatch web site is a fairly effective tool at tracking linux distributions no matter how small or short-lived they are. I think the site's online tool is actually the best one so far, and it has, in my opinion, produced the best stats concerning Linux distributions, unless you consider the NetCraft surveys but these are confined to the web servers' world. Distrowatch takes all of them, servers, desktop, set top boxes, and now has come to include OpenSolaris and BSD Unixes. It spans over 100 distributions, although there are more (but then these are dead or fairly underground distros).