Tools to Gather Data - For $29, you get to have someone run a series of test actions on your site with a video/audio feed of their browsing activity! Awesome tool that we use A LOT.
Crazy Egg - Figure out where all those mouse clicks are happening on your site. Gives great insight into areas that Google Analytics misses out on.
Facebook Insights - A must use when to track usage of your facebook fan page.
Kampyle - Great for gathering actual feedback from customers that are browsing your site. Good to get some actual customer feedback.
Google Analytics - A must have tool for every website owner to measure site metrics like traffic, most popular page etc
Mailchimp - Everyone needs an email marketing tool which gives you information about each campaign you send out. Open rates, which links are clicked, and most effective. Mailchimp is one such platform and is very cost effective too.
Coremetrics - If you can afford it, worth having a 2nd analytics tool installed to compare against Google Analytics data and also because there's a heap of other custom functionality and neat things you can do which Google will take a while to roll out.
Magento - The leading open source ecommerce platform that is a great combination of flexibility, scalability and beautiful design. Reports from your ecommerce tool and the ability to extract data from it is super important. We love Magento :)