It's late and I'm not going to do your homework for you but go check out hotscripts.com - I've seen some so called "light weight" flat file based blog scripts there.
The permissions issues and problems down the road make using just wordpress or wordpress MU easier in the long run though. Plus you get all those extra goodies like the xml-rpc libraries, pinging, comments, etc... all done for you. It's sort of like someone promoting your site for you while you post.
If your going to blog - use wordpress.... or even b2evolution (it's a pain in the rear, but it seems to rank better than WP for me at least.)