There are thousands of Wordpress Plugins available which do all sorts of awesome things. Here are the five plugins which I use on every Wordpress site I set-up or administer:
All in One SEO Pack
This is one of the most popular Wordpress Plugins and for good reason. Search Engine Optimization (SEO) is essential in improving the chance of Google, Bing, Yahoo etc. sending visitors to any Website. With just a few mouse clicks the All in One SEO Pack automates SEO activities such as creating page titles, creating and filling the 'description' meta tag on each post and much more.
Contact Form 7
Contact Form 7 is just so simple to use and it works beautifully. Adding fields or questions to a form is easy and can be done in no time at all. This Plugin doesn't have some of the more advanced features of some other Wordpress form Plugins however what it does provide will suit 99% of requirements.
WordPress SQL Executioner
This is a basic no nonsense and incredibly powerful Plugin which is essential for anyone frequently running SQL queries. This Plugin allows for the running of a SQL query from within Wordpress itself which can save significant time over accessing and using MySQL directly (or using associated tools such as phpMyAdmin). Although this Plugin hasn't been updated for more recent Wordpress versions, it works on installations of Wordpress 2.8.1. which I use.
Google XML Sitemaps
Automate the creation of a Sitemap.XML (or Sitemap.XML.gz) file for your Site. Among the many options is the ability to notify Google, Bing, Yahoo and Ask when the Sitemap has changed. Setup the various options on first Google XML Sitemaps activation and this reliable plugin will take away the worry of creating up to date Sitemaps.
Akismet
Want to avoid huge numbers of spam comments? Then get this plugin. Set-up and forget; Akismet catches almost all comment spam.
I have used all of the above Plugins extensively including on Wordpress 2.8.1. I thoroughly recommend installing and trying out those as listed above!