We've noticed that a lot of our customers use the WordPress blog platform. We use WordPress ourselves and like it a lot, so here are a few of the most popular plug-ins for it.
Popular plug-ins
Akismet Anti-spam – An efficient anti-spam plug-in for your blog that blocks most spam comments. To install it, you'll need an API key, which you can get by registering for free at WordPress.com.
WordPress DB Manager – Back up your WordPress database easily and quickly. You can also optimize your database without using phpMyAdmin or other database tools.
PHP Exec – Makes it possible to run PHP scripts in one or more places on your blog, where PHP is otherwise not accepted.
Extreme Video – Insert YouTube or Google Videos in your blog. The newest version also lets you upload your own videos in .avi, .mpg, or .mov (QuickTime) format.
Social Bookmarks Bar – Makes it easy for your visitors to add your blog to Digg, Technorati, Reddit, and many other social networks.
For more plug-ins, take a look at wordpress.org/extend/plugins.
Update 2011
Some of these plug-ins are no longer maintained, or have been replaced by newer plug-ins. Use the links above at your own risk.