Now that the Google keyword tool is no longer available to the general public, below are given some alternatives:

• https://freekeywords.wordtracker.com/sign_in/
• Good Keywords v3 - Keyword Research Management Software for Windows
• KeywordDiscovery.com
• Keyword Combiner (this is not keyword research tool, but is helpful for sorting your keyword lists)
• The Free Keyword Tool from WordStream - Free Keywords for SEO, AdWords, & Blogging (this is an awesome tool to get hundreds of long tail keywords related to your niche)

Hope you find this information useful. Google Keyword tool may still be free for advertisers, but you need to set up an Adwords account.