The usual WordPress plugin round-ups featured on blogs seem to always feature the same plugins. Here's my list of the lesser-known plugins that I've found useful, as well as how I use them.
aLinks: If you find yourself frequently linking to sources, you can use this plugin to automatically convert text into links. For example, everytime you typed "General Motors", it would add a link to http://www.gm.com to that text, to display General Motors. It can also display an external link icon as well, as you can see with Flickr.
Blogliner: I pull in data from several of the webapps that I use to provide content for my blog. This one can take a specific folder from your public Bloglines subscribed blogs and display them as a blogroll in your sidebar.
Comment License by Alex Reynolds: This plugin is intended to allow you to specify the content license for comments, as in "you own you're own comments". I use it to display my commenting policy, as in when I may edit or delete comments or how I handle abuse.
Database Manager: This admin interface gives you more power to download, delete, restore, and optimize backups of your WordPress tables.
del.icio.us daily post fixer: The default output for the daily linksposts sent from del.icio.us is "daily links for XX/XX/XX". This plugin allows you to specify a different, more appealing title. You have to edit the plugin directly, but it does clean up the title.
deUncategorize: One of the shortcomings of the default WordPress install is that the default category "Uncategorized" remains checked even if you select other categories. This plugin fixes that.
Editor Monkey: You might think that the default WYSIWYG editor for WP is somewhat lacking – I originally did this because the older (2.X) installs editor didn't work with Safari. FCKeditor did (which is one of the two choices you get with Editor Monkey. In my opinion, it's also easier to modify TinyMCE and FCKeditor than the inline editor.
Enhanced-Management: This one is tough, because so much of what it does is transparent. Basically, it imporves the performance and utility of several of the features in the post management, categories, particularly regarding selecting and deselecting items and item deletion.
Feedburner Feed Replacement: I had installed a plugin named "Live" that showed you, er, live hits on your site. I noticed I was getting hit with the various RSS feedreaders and bots way more than I anticipated, and it was having a disastrous effect on server performance. Although my site still loads slowly, by forwarding my feed through Feedburner, I cut the query-time in half.
Footnotes: If you are thinking of academic blogging, this might be for you. More on that subject later.
Share This by Alex Reynolds: This is a feature, replete with it's own chicklet, that cleans up the social bookmarking experience – rather than having the line of icons below your post, you get a "Share This" icon and text, which opens with the various social bookmarking features present plus gives you the option of emailing the post to a friend.
Kill Preview 2: Does exactly what it says – saving time when editing and saving posts.
Viper's Plugins-Used: This plugin provides a table that you can either put in a post or a page that displays all of the plugins that you have installed.
Viper's Video Quick Tags: This plugin integrates icons into TinyMCE for the quick embedding of AVI, QTR, YouTube and other video sources into your post.
WordPress Reports: This plugin will take your Feedburner and Google Analytic stats and display them inside your admin interface.