Related posts plugin : this is good for both your visitors and the search engines. It automatically finds related posts and displays them in a list - very useful when visitors arrive on a page looking for something and they get more info on the same subject.

Metatags plugin: you simply can’t let the search engines handle the text your visitors see in the search engine result pages. You can influence your click-through rate very much just by handling this one aspect of your copy.

As for the search engine optimization aspect - the description metatag can move you up a spot or two when you are in the first 5 with small to medium competition. And it only takes a minute!

Sitemap Generator Plugin: for a small site this wouldn’t be a problem, but when your page count goes over 50 indexing and maintaining a sitemap the old fashioned way just isn’t viable.

This plugin is actually more valuable because it creates both an XML sitemap (like those used by Google) and a normal, HTML sitemap for your visitors.

Some of its features include: support for multi-level categories and pages, category/page exclusion, permalink support, what order to list items in and many other options.