Elise Bauer does yeoman work trying to determine market share for various blogging tools. The methodology has obvious flaws, e.g. conflating Google indexed URLs with individual sites, but the relative shares of the various tools seem qualitatively in line: LiveJournal and Blogger on top, many other well known tools an order of magnitude less penetration.
Well, other than TypePad having twice the share of MovableType. That one seems a bit odd to me given how much longer MT has been around than TP. I'll profer two possibilities. One, WordPress really is putting a dent into MovableType's installs. Two, the numbers are also significantly capturing a usage factor, and the most enthusiastic and consistent group of posters could be on TypePad.
In the comments, Anil Dash asks for suggested improvements. Write a crawler that uses the Google API to generate a seed set, cluster URLs into individual sites, come up with some heuristics to identify the blogging tool, and let it loose!! For bonus points, do some activity analysis to determine if the blog is alive or abandoned.
This one will be kicking around the blogosphere for a while.