Keyword Discovery offers a paid search tool that is quite robust. They offer a free version which is limited in features but still quite usable and helpful. The paid version I have not used, but the features look like you can really get down and dirty with your research, looking at keyword by niche, showing the most popular searches in a particular industry.

http://www.keyworddiscovery.com/search.html

Wordtracker is the other popular paid keyword suggestion tool, and their free version is quite useful. you can only enter one word instead of comparing multiple words. Wordtracker offers a free trial for their more robust product for 7 days.

http://freekeywords.wordtracker.com/

SubmitExpress has a free tool called Keytracker that gauges the traffic of words, comparing those two popular keyword tools. You can enter more than one keyword, and It makes suggestions and shows the traffic for suggested or similar words as well.

http://keywords.submitexpress.com/keytracker.php

For keyword suggestions, Google offers its keyword tool for free, which will suggest keyword combinations based on either phrases you enter, or will analyze an existing web page. Then it does show the search traffic for those words. Another helpful free tool, recommended.

https://adwords.google.com/select/KeywordToolExternal

Microsoft’s Adcenter offers the Keyword Forecast tool. It will compare traffic for a few keywords in graph form, and does not require you to type in letters verifying you are a human, saving time for a quick look. The data seems to be about a year old though:

http://adlab.msn.com/Keyword-Forecast/Default.aspx

SEO Book offers a keyword research tool that compares data from many sources in one view. You can get lost in this one. You can only enter one word at a time, and it does not make the best suggestions compared to the others, but the info correlated is very robust, linking to the different keyword tools and search engines that it used to populate the info, showing the other suggestions and traffic from search engines. Fun toy.

http://tools.seobook.com/keyword-tools/seobook/