These days the correct vanity project is yet another useless ORM.
-- Matt S. Trout on the london.pm mailing list
Vanity projects used to be templating systems, remember? That was the obligatory small project a beginner ought to write for himself. Seems that we move on to higher abstractions.
However, I still think that there's room for a good open source OODBMS. That would be an interesting project. Maybe an interesting vanity project, even!