Google trends -
http://www.google.com/trends
Do search trends follow sales trends? I suspect that they broadly do...
For example, if you search for "music CDs"
http://www.google.com/trends?q=music+cds
you can see that searches tend to "spike" in the lead-up to Christmas. I've sold CDs and this matches my experience - the lead-up to Christmas is a good time to sell music CDs. (Of course, the overall trend is shown too - that searches for music CDs are going down and down with time... As more people buy downloadable music instead!)
Knowing these sales trends could be useful if you're planning to get into a business selling a particular product! For example, if you're going to sell "business reports" - here's the trend...
http://www.google.com/trends?q=business+report
As you can see, searches plunge in about the month before Christmas... Then jump back up again at the end of December. In my experience, sales of "business reports" tend to do the same thing...
You can see a similar trend in the search for "entrepreneur" - if you sell "entrepreneurial" products...
http://www.google.com/trends?q=entrepreneur
(I think the reason is - people don't usually give "business reports" away as Christmas presents. And in the lead-up to Christmas, what to buy as a Christmas present is what's on people's minds...)
Anyhow, knowing these trends could be useful - so you don't waste your money advertising heavily at times when demand is low anyway...
Cheers,
Dien