If you have a point of reference, I find Google trends useful as well. By point of reference, I mean you have some keywords where you know the actual click volume from personal experience, i.e. you rank 1st on that keyword.
I personally think adwords tool is the best freely available tool right now. Spyfu and wordtracker are other options - paid. More expensive Hitwise reports, can't beat these in terms of depth and accuracy.
(From google adwords tools).
For comparison, "people search" gets 68,000,000 January, and average of 30,400,000