SeoBook Tool--- Aaron Wall has tons of other
free tools that rock.
It's free, runs off Wordtracker (which is a paid service), and has a new export to csv option.
Export to Csv is really nice because in a couple clicks I can print keywords and put it together in a notebook with all my other project files. I find I'm much more productive when I take the time to put together a notebook of notes, keywords, copy, etc...
BUT.... remember that your brain is the best keyword tool. The tool won't spit out all related keywords and do 100% of the work for you. You have to think out your keywords beforehand and then use the tool.
For example: "Mortgage" doesn't return "financing" or "leasing"... yet they are essentially the same thing. Understand?
This is probably one of the better blog posts I can think of off the top of my head that explains this in more detail.