Free KeyWord Software Tool
KWmap.net is an interesting free tool for keyword research using a graphical representation to show how different terms relate. So it would be useful for building silo sites for optimum linking.
However, it does suffer from the weakness of not providing any statistical data with the keywords. So it becomes impossible to target your marketing efforts according to whether you want to hit high volume keywords for an Adwords campaign or maybe long tail keyword phrases for article marketing.