Google Adwords seems to be the popular choice. i think it's because Google is the number SE
Google's keyword external tools is more popular for this task. Another one is wordtracker is also good. Generally most of SEO Analyst use these tools. There are some more tools available in which some are paid and some are free.
The best keyword research tool is Google's keyword tool, which is widely used by all the webmasters for doing keyword research activity. Google had developed this tool mainly for their Adwords campaign but now all the webmasters are using it for generic keyword research.
There are many other tools also available but Google's keyword tool is far better as compared to others.