Free Tools

http://inventory.overture.com/d/searchinventory/suggestion/

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

http://www.nichebotclassic.com/

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

http://www.google.com/webhp?complete=1

http://www.digitalpoint.com/tools/suggestion/

http://www.kwbrowse.com/

http://www.goodkeywords.com/

Paid Tools

http://www.wordtracker.com/