I agree that Google Keyword Tool is the best, with some configuration - you can easily get good suggestions
The google keyword selector shows the optimal longtail keywords within the 2000-10000 searches / month range.
A method that I use is google keyword tool. I dissect the keywords within the niche. I go by competition, how long the keyword is, and the exact matches. Google keyword tool is a great free suggestion tool.
i heard keyword scout is great, many of my friends use it but don't have any money to buy it now so I stick with my stealth keyword competition analyzer (SKCA)
googles keyword tool plus use googles suggestion tool.. when doing a search in google it should give you suggestions..