Numerous sites use plugins to expand functionality offered by WordPress. A lot of good plugins emerge and fight for a place under the sun. For various reasons I have worked with a lot of plugins and would like to make a public list of plugins I have found to fit my needs.
Very good if one uses a lot of plugins as greatly reduces the time needed to spend updating other plugins.
WordPress, version 2.9 introduced batch updating plugins thus diminishing this plugins usefulness, but still - it's a good plugin.
Comes along with your default Wordrpress installation, to enable one must get a key (free and easy to add). Reduces spam comments, standard plugin to fill this need for hundreds of thousands WordPress users.
Hyperlinks have this unfortunate tendency to change and your links can be outdated in no time. This plugin checks links inside your posts, notifies if some of them don't work, allows to change them easy too. There may be false positives, but one can exclude them easy enough, and include them easy enough too.
Sometimes one needs to have custom look for his pages: one post, but different looks for various parts of it. It's accomplished using css, bt even css needs something to be attached to - div tags or something like that. Now that (while possible) is bad for non tech savvy WordPress users. This plugin (utilizes WordPress custom fields, means exporting posts exports this information too) allows user to get that functionality. Note that developer still needs to add custom fields to theme. More information on how to do that can be found in WordPress Codex. Overall
<?php $meta_values = get_post_meta($post_id, $key, $single); ?>
While not the most user friendly (the list is hard to keep track of as it doesn't allow meaningful titles), it allows to change background looks easy enough even for non programmers. Note that sometimes your theme may need some changes to make unique identifiers for pages / posts. I'd suggest post/ page ID. Like
<body class="post-<?php echo $post->ID;?> outercontainer">
Plugin author should make notification that this plugin works with PHP 5 only more visible, but overall very good plugin to add breadcrumbs to your blog. Easy tweaking to fit ones needs.
Very good contact plugin, it allows various instances / functionality tweaks. While (at the moment) their documentation says that it's not possible to use it out of editor panel (in theme files, for example) it is - use something like this:
<?php echo do_shortcode('[contact-form-7 404 "Not Found"]'); ?>
Most of the functionality offered by this plugin will be not be understandable for non-geeks, some of functionality (enabled by default, but easily disabled) will clog your Posts => Edit panel. But overall this plugin offers a vast list of possibilities for tweaking your blog. Starting from security and ending with meta tags.
Improves security, easy and simple.
Plugin for all your image administration things. The best one. Multiple plugins allow to extend it's functionality even more, easily adaptable to work with other scripts too. It's taxonomy functionality could be improved allowing multiple levels, but still: administration is really easy, functionality vast.
While using this plugin will definitely break your site W3C compatibility (usually broken by posts copied from MS Word already) it allows easy addition of tabs to your posts and pages. Note that pPart of W3C errors it creates could be easily fixed by it's author.
Very good if you want your site to have multiple languages in it. But, after you add it to your site, you can't disable it (at least now). If you do, all those posts in different languages show up in the same post - very messy. And I hope they don't change syntax for adding custom translations anymore, as doing so can force one to change his theme files again.
Will work on hosting with htaccess files (usually Linux/Unix) only. But really easy to redirect links in case of need. Best used together with Google Webmaster Tools or some such software - it reports broken links, Redirection plugin fixes them.
Good looking and not intrusive way to ask your readers to share your post / page. There was this one update what broke the plugin in some cases, but overall very good work. Easy administration too.
Checks for statistic links in themes, shows them: easy way to check for malicious links. It would be good if it added notification to dashboard if suspicious links were found, but overall - easy to use, descriptive. Tested in real live situation, found malicious links correctly.
Needs key (remember Akismet), but, if you don't want Google Analytics or some such software, very good and descriptive software for collection your reader stats.
While this plugin can easily be used for nefarious needs (getting content from other sites, displaying it as your own), it can be very good if you need to get info from your other sites who doesn't use WordPress as their platform too. Caching (allowing to save server resources), multiple instances.... Take notice that it may need theme tweaking too.
It does what it says: shows related posts. Works on Windows and Linux servers, easy administration.