I made this because I'm a nerd who goes to conventions and who lives in North America.
Each circle on the map represents an annual nerd convention. The size of each circle corresponds to the convention's yearly attendance, and the color indicates the main theme (science fiction, comics, boardgaming, etc.) of the convention.
If you're looking for a geographical representation of nerd conventions, Conmapper is (as far as I know) the best one on the Internet.
As I discovered during user testing, people looking for a convention to attend don't actually need to see them represented geographically. In other words, a map is not the appropriate interface to this particular data set. A user is more likely to want to see a list of conventions in the next few months, or a list of upcoming conventions near their current location. Appropriately, Conmapper led to Happening, which attempted to solve the same problem with a different interface.
I tried to write my own dynamic color generator despite not knowing anything about color theory, so the visual presentation is just plain ugly. It's also not usable by some users with color vision deficiency.
Think hard about the visual noise that a feature will introduce before adding it. In this case, representing attendance numbers through the radius of each dot was a cool experiment in information design, but it didn't actually help testers decide what conventions they wanted to go to.