Most folk refer to quantcast and adplanner when it comes to demographic research, and Google Insights for checking out seasonal trends.
But there is a 4th tool you should add to your toolkit.
http://clues.yahoo.com/#q1=iphone&q2=ipad&ts=0
Why?
1) Hot trends

2) Keyword Demographics

3) Geo targetting

4) Search funnel and related terms

5) Week parting

6) Day parting
