Akismet — the only way to stop spam comments from ruining your blog. Akismet is very accurate and works like a charm.

Google Analytics for WordPress — I used to cut and paste the Google Analytics script into the footer of my blog, but then you have to worry about it every time you change your theme. By using a WordPress Plugin, you can change themes without worrying about changing the Google Analytics script.

Google XML Sitemaps – This is a very handy WordPress Plugin not just for creating the XML sitemap every time you make a change to your blog, but also for notifying the major search engines of the change.

All in One SEO Pack — It takes a little work to fully put this plugin to use, but it is very helpful in allowing you to set the title tag for each post and each page, including your main page. The title tag seems to be the most important one for search engines in identifying what a page is all about, and this plugin lets you tailor the title tag as well as other parameters.

What about Google AdSense or other advertising-related plugins? I usually put the AdSense script in a text widget, so I don’t use a plugin for this.