Google's AdWords Keyword Planner tool is another common starting point for SEO keyword research. It not only suggests keywords and provides estimated search volume, but also predicts the cost of running paid campaigns for these terms. To determine volume for a particular keyword, be sure to set the Match Type to [Exact] and look under Local Monthly Searches. Remember that these represent total searches. Depending on your ranking and click-through rate, the actual number of visitors you achieve for these keywords will usually be much lower.
If you already have a keyword list, the Free Keyword Grouper can transform your list (up to 1,000 keywords) into an organized keyword structure ready for high-performance PPC campaigns and an SEO-friendly information architecture. Just paste in your list to get back keyword groups in seconds. This keywords organizer tool will help you sort your list of keywords into structured, workable groups that can then be used for easy ad text creation. The keyword tool can also be used as an SEO keyword organizer to assist in site design.
Long Tail Pro is a keyword research tool that helps you quickly find low competition keywords for your website. In particular, the tool is built for people doing SEO (search engine optimization) as opposed to people doing paid ads (Google Adwords PPC). I'm sure there are some advertisers that use the tool for paid ad keyword research, but the tool was built for SEO keyword research.