Keeping SEO in mind, here are a few free tools for conducting keyword research:
• Google AdWords: www.adwords.google.com
• Google Trends: www.google.com/trends
• Microsoft adCenter Labs keyword forecast: http://adlab.msn.com/Keyword-Forecast/
• Trellian’s free search term suggestion tool: www.keyworddiscovery.com/search
• Wordtracker’s free keyword suggestion tool: www.freekeywords.wordtracker.com