Use the Google Adwords Keyword Planner to find new and related keywords, but ignore the search volume data! The search volume data in the planner is really only useful for keywords that you’re actually spending money to advertise on. Otherwise, these volumes are not reliable. While not really helpful to decide which keyword is most used by your potential audience, Google Adwords Keyword Planner makes a useful tool in coming up with ideas for potential keywords!
Taking this site as an example, “home working” is a pretty important keyword. I’d love it if I could be at the top of Google for that phrase. However, despite pouring heart, soul and a great many man-hours into creating a useful resource for home workers, I’m not yet anywhere near the first page of Google’s search results. (I’m actually languishing depressingly on the 11th page of results at the time of writing!)
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.