Image courtesy: si1very

I found these very interesting keyword suggestion tools which are very important in SEO. I thought that you might be interested. You can find more details by visiting those sites.

• Free: SEO Book keyword suggestion tool
  (http://tools.seobook.com/general/keyword/)
  
• Free: Google AdWords keyword suggestion tool
• (https://adwords.google.com/select/KeywordToolExternal)
• Free: Google AdWords traffic estimator
  (https://adwords.google.com/select/TrafficEstimatorSandbox)
• Free: Google Trends
  (http://www.google.com/trends)
• Free: Digital Point keyword suggestion tool
• (http://www.digitalpoint.com/tools/suggestion/)
• Free: MSN Search Result Clustering
  (http://rwsm.directtaps.net/p5/clustermain.aspx)
• Various Prices (approximately $7 / day to $250 / year):
  WordTracker (http://www.wordtracker.com)
• Various prices: Keyword Discovery
  (http://www.keyworddiscovery.com/)
• Keyword discovery also provides free limited depth usage and has a free keyword directory
  (http://www.keyworddiscovery.com/keyword-directory.html)
• Various prices: Keyword Intelligence
  (http://www.keywordintelligence.com/)
• Free: Overture Search Term Suggestion Tool
• (http://inventory.overture.com/d/searchinventory/suggestion/)
• Free: Google Search Suggestion Tool (http://www.google.com/webhp?complete=1&hl=en) (http://tools.seobook.com/general/keyword-information/)
• Free: Google Sets
  (http://labs.google.com/sets)
• Free: Good Keywords
  (http://www.goodkeywords.com)
• Free: Ontology tool
  (http://www.gorank.com/seotools/ontology/)
• Free: Quintura
  (http://quintura.com/)
• Free: Keyword Typo Generator
  (http://tools.seobook.com/spelling/keywords-typos.cgi)
• Free: Lexical FreeNet
  (http://www.lexfn.com/)
• Free: Tag Cloud
  (http://www.tagcloud.com/)
  
• Free: MSN AdLab
  (http://adlab.msn.com/)
• Free: SpyFu
  (http://www.spyfu.com/)
• Paid: KeyCompete
  (http://www.keycompete.com/)
• Free: Competitors websites and related search suggestions from various search engines such as Vivisimo, Snap, Gigablast, and Ask.