I would say the one on this very site is the best, and it's free: http://www.digitalpoint.com/tools/keywords/
DP keyword tool is great but if you need something prettier to send to clients or other people, I recommend WebCEO. There is also WebPosition and IBP in the paid category. The cool thing about DP's tool is that it doesn't use any of your computer's resources as it runs on the server.
I use WebCEO regularly. It is great for tracking visitors, because with its HitLens (web analyzer) you can get any kind of information about your visitors that you can imagine. It is really necessary to know how your visitors come to your page, what pages are interesting for them, how long time they spend on your pages, how your PPC campaigns work, what is your ROI and much much more; you can even know what browsers your visitors use, what are their navigation paths, how many visits your customers need before purchasing something from you and so on. I find this tool very accurate and kind of a must for an internet marketer