I am currently using the following plugins on this blog, I find them all useful.
Akismet – By Matt Mullenweg. Spam protection that comes with the WordPress download.
Challenge – By Lordchaos. Restricts comment spam by asking the commenter a math question challenge.
Advanced-Admin-Menus – By Andy Staines. This plugin creates a drop down menu that lets you navigate directly in your WP Administration panel pages without having to load any interim pages.
Simple Recent Comments – By GNot. Displays excerpts of the comments most recently posted in your blog.
WordPress Database Backup – By Scott Merrill. Plugin that makes backups of WP database easier than using phpMyAdmin.