First of all, a definition of Tumblelogging from Ozimodo's website

Tumblelogs are quick-and-dirty. They are loosely structured and used to share various iotas of interest. Throw a link log, a moblog, a quote blog, and a code blog (colog? quoblog?) into a blender and out pops a delicious, fat free tumblelog.

My personal favorite example of a tumblelog is Projectionist and there's the site of the creator of Ozimodo (Chris Wanstrath). There are many others out there.

On to the CMS. Without waxing too wordy about what CMS for tumblelogging should do, I'll provide a quick list and say that Ozimodo does what it should do well.

• Post types - this is the key difference between blogging and tumblelogging. With a blog all posts are, well, posts. With a tumblelog, a post could be a photo, a quote, some code, IRC or IM snippets or anything else you can imagine. This is handled nicely in Ozimodo.
• Tags - it has tags.
• Flexible templates - If you're going to do much customization of your Tumblelog, you'll definitely need to know some HTML and it would be nice to understand a teeny bit of Ruby to use the rhtml templates. None of the configuration for Ozimodo is done in the admin interface, it's all handled with .rhtml template files.

Ozimodo is written in Ruby on Rails and, like pretty much every Ruby on Rails app it is simple to install. The process is download, create empty database, edit 2 lines in config/database.yml, type 'rake migrate' and you're done.

Attached to this article are several screenshots of Ozimdo in action, including the admin interface. It comes highly recommended as a good (and the only) niche CMS created for Tumblelogging.