There be WordPress stuff here for the taking! Instead of remarketing them all here, I'll just point to my WP Plugins page where you can read about them:
My non-plugin yet still playing in the same ballpark with WordPress endeavors:
Turn that lowly Trackback link on your blog into something more: a popup window which displays the Trackback URL to your readers.
A PHP script providing a fast and easy way for developers (or anyone, actually) to display the source from a plugin, with syntax-highlighted code. Update [Jul-4-2005]: Security fix.
One great thing about custom permalink structures is, you can use more than one. I've implemented a foreshortened version of the regular permalink format for use on my blog. There are some limitations (and problems) with it, but it accomplishes what I was after. Explanations with some of the reasoning behind its purpose at my blog.