Market Samurai is the leader in keyword research. It blows away Google Adwords and any other website when it comes to really nailing down the numbers. The software is based on Adobe Air, so it will work on Windows and Mac. They do a great job of giving you in depth video tutorials of each section, they also have a really good newsletter explaining new features and general SEO topics. You're allowed to put this on 2 different machines, so if you have an extra hand at your company and you need to give them something to do, have them learn this software and really build you an amazing keyword list.

One of the best parts about this software is that you buy it once, and you're set. They're always updating it, and there are no recurring charges. I've heard their other item, Domain Samurai is really good too, however I haven't had time to check it out.

Note: They have really bad tour pages, filled with information. Ignore it and trust me when I say that this software is legit. I'm attaching some of their instructional videos at the Pro Tips section.