Here is a list of some of the best Wordpress Plugins that I use on my sites. Most are very easy to install, just download and drag them into your plugin folder and you are ready to go.

All in One SEO Pack
This is one of the most essential plugins for wordpress. It really streamlines the process of setting your wordpress site up to make it extremely SEO or search engine optimized.
Download All in One SEO Pack

Contact Form 7
Easily create a simple contact form on your blog so users can send you emails without knowing your email address. Offers various languages and comes with a Captcha function to help ward off evil spammers!
Download Contact Form 7

Google XML Sitemaps
Create a Google compliant sitemap for your blog.
Download Google XML Sitemaps

Smart YouTube
Easily add Youtube videos into your blog post, comments, and even RSS Feed
Download Smart YouTube

Google Analytics for WordPress
Easily integrates your Google Analytics code into your wordpress blog. Track visitors and stats of your blog.
Download Google Analytics for WordPress

AdSense Manager
Simple plugin that allows you to dynamically put in Adsense into widgets or posts.
Download AdSense Manager