The Wordstream Free Keyword Tool is a nice alternative to Wordtracker's free tool.

You can punch in a keyword and it will suggest related keywords. You can get 100 keyword suggestions for your keyword, or for your keyword and the related keywords you approve.

The tool shows Relative Volume as a bar graph on the screen, but if you click the button to copy the results to your clipboard you will get them as raw numbers.

I don't think this will replace any tools in my keyword research toolbox, but I do think it will make a nice addition.