Blosxom is a very lightweight, yet still featurefull weblog system. The whole thing comes as a single perl script which you place in a cgi-bin dir and edit. Posts are simply text files with the title on the top line. Catagories are the directories containing the posts (which can be nested), and formatting is done using flavours: write an html flavour for web browsers, an rss flavour for aggregators, etc.
Blosxom also has a simple plugin system, which can be used to extend its capabilitites greatly.
More info at http://www.blosxom.com/