With all the others mentioned, it seems a shame to leave out LiveJournal.

They offer the code under a variety of licenses (GPL primarily), and operate a commercial site hosting free and paid journals using their code as well. Apparantly it supports easy hooks for Perl extensions, is coded in Perl on MySQL.

LiveJournal docs are here.

I've never used it, but it appears to be supporting a large user community on their own site.