Compared to the other tools in this list, Übersuggest is the tool I have used the most. To use it, it is not necessary to create an account. Basically, Ubersuggest provides you with a list of keywords based on auto completions of the primary keyword you would have entered in the search box.
KGen is one more tool that could help generate keywords. Installing this Firefox plugin is normally necessary before being able to get keyword ideas. After the plugin is activated and visible in your Firefox sidebar, visit a page related to the niche you want to research and click on the "Scan It!" button. That way you may have a list of keywords generated based on the page you have visited.
Bing Webmaster Keyword Tool
Until recently, I was not aware of the fact that Bing Webmaster had made available a keyword tool that happens to also provide metrics. Before using this application, you may have to get a free account. It is after I verified the ownership of my website that I was able to see the research Tool in my sidebar (find "Diagnostic & Tools", then in the drop down click on "Keyword Research")
SEO Book Keyword Suggestion Tool
One more keyword research tool is Keyword Tool by SEOBook. Keyword Tool is powered by Wordtracker and it can produce a list of keywords with metrics. You will normally need to create an account before being able to use the SEO Book Keyword Suggestion Tool.
Wordpot does not require an account. In fact, the keyword search box is located on the home page which simplifies things in my opinion.