Akismet – Considered essential, disposes of spam coming to your site/blog, very efficiently. Action required to use it: configuration just requires you register for an askimet key, register for an askimet key here: http://akismet.com/get/
All in One SEO Pack – Creates search engine friendly page titles and descriptions. Highly configurable and ability to drill down to unique blogs to configure your meta information and page titles.
Fast Secure Contact Form – A great feedback form that requires minimal setup. Add the plugin and include the short code [si-contact-form form='1'] on your contact page.
Google Analytics for WordPress – A simple way to incorporate google analytics (hits, traffic sources, etc) to your website.
Widget Logic – This little plugin allows you to control when widgets appear in your sidebar or other widgetized areas.
Simple Facebook Like Widget – Automatically adds the facebook like button to your pages and posts which allows your visitors to link to your site from their facebook profile.
Newsletter – Offer an opt-in newsletter to your visitors with newsletter composer, highly configurable.
Website Diary – I use this plugin to document design or management work on the website. Could be used for any notes you would like to keep. Access it from your admin dashboard.