The Search feature on the Kindle is surprisingly powerful. You can use it to search for items your Kindle, the Kindle Store, Amazon, wiki, Google and the built in dictionary. It also lets you access the internet without going to the Experimental section. Just type the website you would like to go to in the search field and use the go to web option.
The Kindle has a built in calculator that you can also access through the search menu. Type out the equation in the search field and then hit enter. The calculator can do basic math and trig functions. It even allows you to use parenthesis for more complex equations and groupings.
You can access Search by pressing the Menu button.
Advanced Tip: You can use shortcuts to access the search engine from the Home screen. I’ve been told you can also use the search feature to find multimedia files on your Kindle. I haven’t tested this out yet.