Recommendations wanted for lightweight, hackable blogging platform

[Jeremy Henty]

I've always put off starting my own blog (despite getting the domain a
couple  of years  ago) but  now it's  impossible to  remain silent[1].
But, being a geek I insist on hosting it myself rather than signing up
to blooger or  wordsmush, so I'm looking for  a blogging platform.  It
must be small  and hackable so I  can get my head around  it.  It must
support tags  and categories.   I don't care  about themes  and bling.
I'd prefer it to be database agnostic and written in Ruby, but neither
of those is  mandatory.  I have root on my own  virtual host, so there
are no restrictions on what I can do to install it.  Any tips?

Thanks in advance,

Jeremy Henty

[1] http://www.boingboing.net/2009/11/20/britains-new-interne.html