List of my 5 favorite Wordpress Plugins.  All  five added functionality that I felt my site lacked without them. 

Plugin Name: Adsense-Deluxe 0.8
Function: Places Google AdSense ads in your WordPress Posts. Requires WordPress 1.5 or higher.  Simple to use and I’ve never had an issue with it.
Author: Acme Technologies.

Plugin Name: EasyRedirect 0.2
Function: Plugin to easily redirect to another page with javascript.  I use it for landing pages or to send people to my forums on another domain.
Author: By Paul Bain.

Plugin Name: Google XML Sitemaps 3.0.1
Function: This plugin generates a sitemaps.org compatible sitemap of your WordPress blog which is supported by Ask.com, Google, MSN Search and YAHOO. Configuration Page.  Simple to use.
Author: By Arne Brachhold.

Plugin Name: Gregarious
Function: Social bookmarking.  Makes submitting your pages to Digg and other sites easy for you and your readers.
Author: By Ankur Kothari.

Plugin Name: Search Pages 2.3
Function: This makes search queries look at pages and posts instead of only posts.  I wanted an easy way to search for pages that I didn’t link to directly.
Author: By InternetOfficer SPRL.