Google keyword search is the one I use most often. It is easy to use and you can display or hide columns as you please. they even have a tool that will search you webpage and give you suggestions for keywords related to your page.
I think a great point to make here is to not use just one keyword tool when conducting keyword research. If you need something quick and simple then ok, but for conducting in depth keyword research you should use a combination of tools. I will often find a great list of keywords using Google Adwords Keyword Tool, but then once researched further with other tools this list will change. Once you get your final list of keywords and the landing pages they will associate with, plug those keywords into other tools to verify you are making the best possible choice. Different tools will often provide drastically different results.
I own keyword elite but never use it anymore since Google added search volume to its already good Adwords Keyword Tool. I still use SEO elite to analyze the backlinks of my competitor...saves me a lot of time. DianeS
I've become a big fan of Market Samurai. If you follow the link from the 30 Day Challenge website, you can get a fully functional copy of it for 40 days.
Very little support for WordTracker? SEO pros like SEOBOOK seem to use WT. Plus I understand WT has some good metrics (KIE, or KEI?), esp. good for finding longtail phrases. All I get from Google Keyword Tool is competition vs volume. Am I missing something?
I use google adwords tool and wordtracker.. Thought it may not be that accurate it just serves its purpose..