Here is one some of you might like. It is free right now, but they are talking as if they might eventually have it as a paid service.
It allows you to use the Overture or Trellian data feed, as well as the graphical term trend from Google.
The Overture Keyword Selection Tool is from Yahoo. It gives related searches that include your term and estimates the number of times that term was searched on last month.
Trellian compiles keyword search statistics from over 180 search engines around the world.
Google Trends analyzes a portion of Google web searches to compute how many searches have been done for the terms you enter. A graph is then shown with the results.
This one is called Popular Searches. Here is a link to it so that you may see if you like it
Popular Searches - Search Keyword Popularity Tool