Sphider: It can provide insight as to which keywords you could target, as well as determine if your visitors are finding what they are looking for as they browse your website.
Magpie RSS Simplified: This plugin allows you to implement MagpieRSS easily within your theme. MagpieRSS is already integrated with WordPress.
WP-Menu: Customizable hook to pull Page Navigation into a theme. Page Exclusion, Inclusion, Site Map, Top Level, Secondary menus and more.
Better Feed: Plugin for WordPress blogs that allows you to enhance your RSS news feed. This plugin will allow you to add anything below each entry in your RSS feed, so you can insert any text link / banner advertisement that you wish.
Sociable: Sociable appends a list of the social bookmarking sites (ie. Digg, Del.icio.us, Reddit) at the end of your posts and allows your visitors to bookmark.
FlikrRSS:This plugin allows you to easily display Flickr photos on your weblog. It supports user, public and group photostreams. The plugin is relatively easy to setup and configure via an options panel. It also has support for an image cache located on your server.
Google Sitemap Generator: If you need to generate a XML compliant sitemap for search engines, Google Sitemap Generator is the best choice (supported by most popular search engines: Google, Yahoo, Ask.com, and MSN)
Adsense Deluxe: Adsense Deluxe is probably the best Adsense plugin available on the web. Very easy to use: first you define your ad code through the Admin options, then insert a HTML snippet ("<!–adsense–>") anywhere in your post content, and it will automatically repace it with your ad code.
Admin Drop Down Menu: This is a must have plugin for every WordPress site. It saves a lot of time and clicks. It basically makes your secondary menu activate on mouseover. For example, instead of click Write, then click Page; you can mouseover Write and click Page (this will save one click and one pageload).
Gravatar 2 Plugin: It basically displays a Gravatar (globally recognized avatar) of your commentators. Automattic (WordPress creator) owns Gravatar.
WP PageNavi: WP PageNavi adds a better paging navigation to your WordPress site. It is nice if you have a lot of posts because it allows your visitors to skip from page to page.
Sideblog: One of the popular WordPress plugins, Sideblog is the best way to post short notes on the sidebar. It gives you options to define the sideblog category, how many entries to display, and whether to exclude from RSS feeds.
Intouch Ajax contact form: It works very well and filters all the spams. NO SPAM at all. It also allows you to customize the email subject, thank you messages, and adds additional form fields
WP e-Commerce: If you want to sell stuffs on your blog? You must check out this WP eCommerce plugin by Instinct. It is an Ajax powered shopping cart, very easy to use. You can setup your shop in minutes through the Admin panels.
Ajax Polls: , it adds a Ajax poll system to your site. It is very flexible with a lot of features. Layout is completely customizable via Admin panels. It also allows you to archive the polls. Highly recommended!
Cforms2: It is not just a contact from, if you need it to be it can be configured into any type of form you can think of. Do not be put off by the functions list, these are understood by the pro,and taken for granted by everyone else, upon using the plugin you will see how easy this plugin is.