Keyword research is an important aspect of marketing. There are many paid keyword tools like nichebot, wordtracker etc. and these costs a nice amount to use.
For people who don’t want to invest in such paid keyword tools can use the following site for their keyword research.

1. Google Keyword Tool

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

2. Google Insights

http://www.google.com/insights/search/

New tool by google which can be used to compare the search volumes of a keyword across diiferen regions and countries.

3. Google trends

http://google.com/trends

Not exactly a keyword tool but gives a list of hot searched topics/keyword in the prevoius hour.

4. Adcentre labs tool by msn

http://adlab.msn.com/

A free tool provided by MSN

5. Wordtracker

http://freekeywords.wordtracker.com/

Wordtracker also provide a free tool for doing keyword research.

6. Good Keywords

http://www.goodkeywords.com

Tool for researching misspelled keywords

7. SEO Book Keyword Suggestion Tool

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

This tool provides you loads of information about keywords

8. KWBrowse.com

http://www.kwbrowse.com

Excellent tool for researching lsi keywords

9. KeywordSpy.com and Spyfu.com
Keywordspy is not actually a free tool but they provide you limited information about your keywords for free. Keywordspy and spyfu together are a deadly combination for doing PPC research.

10. KeywordExtend.com

http://www.keywordextend.com

This is a tool for generating keywords variation for adwords.

11. Spacky.com

http://www.spacky.com

This tool provides search volume for your keyword across google,yahoo and MSN.