Hi, I use wordtracker, it's quick and easy and they also have a trial period available. I hope this helps.
http://www.nichebot.com/ is the best paid tool. Gives you access to other keyword research tools at a fraction of the cost.
Without a doubt, Google's free keyword tool. If you can spare the bucks, subscribe to WordTracker's service. It's obviously not as good as Google for estimating volume, but I find WT's lateral research tool better for finding related keywords.
Google keyword external and the free wordtracker service. Free is good. =) Has anyone tried Keyword Elite?
Trafficmarks had offered the stuff for free during the first release of the product which is not limited. Google Keywords Tool is a free service offered by yours trully google.
I used to use a tool that would take your keyword, get the top search results and then retrieve all the other keywords in the meta tags of those pages. It was great for coming up with new ideas which you can't do with any other tools I believe. Sadly I cant remember what the tool was called