Otto offers a great way to keep track of site-specific WordPress tweaks using a site-specific plugin. I could see this also being useful for a developer or consultant who wanted similar changes across multiple sites.

Replaces the WordPress HTML editor with a nice Markdown editor

Plugin to easily replace the WordPress login screen logo with a custom graphic. No configuration required

Plugin to redirect users to your true URL when they arrive via the DiggBar

Notify via e-mail or RSS feed when a 404 is registered on a WordPress site

A batch category editor for WordPress

Plugin to allow OpenID authentication or comment identification

A simple contact form which can easily be added as a WordPress page

Creates some useful admin-screen reports for those using Google Analytics and FeedBurner

Allows commenters to receive e-mail notifications of followup comments

An easy way of inserting Technorati Tags within posts

Automatically generates a list of related entries based on post content

Lists the most recent comments on the blog, with varying options to control how many comments and how much of those comments form the excerpt

Rearranges the WordPress title tag so that the post title precedes the blog name

Enables alternate formatting to call out comments from a post's author

Provides a nice comment management interface that allows for more features through the admin panel than the default WordPress install

Lets users associate a small picture (avatar) with their e-mail address for commenting purposes

Integrate your FeedBurner feed with your WordPress blog

Removes the nofollow attribute from links left in comments

Anti-spam plugin for WordPress

Shows a list of the most frequent commenters, including links and options

Insecure script to force a reset of a WordPress admin account

Plugin to enable the use of OpenID for commenting on a WordPress blog

Automatically show the advanced feature toolbar in the TinyMCE editor on WordPress 2.1