Good question which I was asking myself recently. This is a neglected corner of the CMS landscape. However, I did find:
- this rather detailed and intelligent summary list:http://www.matthijskamstra.nl/blog/index.php/2011/02/16/lightweight-cms-for-simple-projects/
- a briefer but recently updated list of 40 lightweight CMSs http://www.abcphp.com/out/top-40-free-simple-lightweight-cms-|-vivalogo-resources/
Two other lists, both from 2012 and neither suggesting much examination of the products:
- http://webdesignledger.com/tools/10-simple-and-light-weight-cms-solutions
- http://speckyboy.com/2010/07/19/14-light-and-east-to-use-open-source-content-management-systems/
In case you're wondering, the project I was looking at using one of these for got a little more complicated and I ended up going to a more heavyweight system, with a simplified interface on top of it.
There's a lesson there. Many systems that will cope with complex needs (Drupal, MODX etc) can still be installed pretty quickly, and your users' needs are only going to grow more complex over time.
That said, I'd still like to think lightweight systems have their place. Let us know what you chose.