Distributions and their derivatives

Here is a bigger list of Linux distributions, but it grows larger every day, it seems (but some distributions already "died").
http://en.wikipedia.org/wiki/List_of_Linux_distributions

It is interesting to see the "family tree":
http://upload.wikimedia.org/wikipedia/commons/1/1b/Linux_Distribution_Timeline.svg
(You can see the chronology of development and it's also clickable, so you can visit web pages of various distros.)

There is a site that follows distributions and rank them by popularity - DistroWatch.