I'm looking for some weblog software to use/extend, but all the ones I've looked at so far are terribly written - attempting to work with them is far too effortful and depressing.
I'm looking for something that is well architectured and easy to change - modular code, distinct Model-View-Controller layers, sensible data structure, and so on.
The key is that the core engine must be easily modifiable - I am not looking for software which might have lots of plugins if the core code is terrible.
Also, please only recommend software if you are familiar with the source, and do not simply post what you happen to use for your own blog.
It must be Free Software / Open-Source, and runnable via Linux on Apache HTTP or Tomcat, but otherwise language can be anything sensible.
Whilst I can build my own blog software, I'd prefer to hit the ground running with an existing package, rather than spending time re-inventing the wheel.
(Assuming, that is, that there are any suitable existing wheels!)