Google Keyword Tool - There are lots of keyword tools out there, but this is the one you should use to do the majority of your keyword research. Who else has better quality data than Google? Besides, you're trying to reach people on Google and this tool gives you volumes of information about how your target audience searches on Google.
Google also released a new keyword tool called the Search-based keyword tool. Basically, you enter the URL of a page on your website and a couple of keywords that you are trying to optimize for on that page. Then the tool gives you more keywords that are related to the web page you typed in. Personally, I perfer the classic Google keyword tool.
