I have to admit that I’ve been impressed by Serendipity. It just works. I really like PHP, but very often things are implemented pretty badly with it, and code gets rapidly hard to improve, modularize and so on… but here, it really looks like the developers have done a great job, congrats to them!
Update 2010/04/11 : I wanted to be able to post multi-language entries, and existing plugins for WordPress seemed to do just that. No regrets, thanks for the ride, s9y!