I have just written my second plugin for WordPress, Tagline Randomizer. The plugin maintains a list of taglines and/or quotes in a text file and then randomly selects one to use as your Tagline or 'blogdescription' each time your site is accessed. The taglines are all created by you in the Administration Menu and are stored in an external text file. The text file is automatically created by the plugin so no changes are made to your database nor is any editing necessary to your theme.
You can see it in action and read about it here, http://dailydoseofdave.net/tagline-randomizer
Tagline Randomizer joins a host of other tagline/quote plugins but approaches its goal in a different and more simple way than most.