NOTE: I’ve been staying in the beautiful Cumberland Lodge just outside of London for the past few days and have had very limited access to any sort of internet connection. I had just enough time to upload this brief post before my meetings today. Thanks for bearing with me.
Google Insights for Search is a great little tool to help gauge interest.
Here are the highlights. First, point your browser to: http://www.google.com/insights/search/
Type in a search term that you might use if you were searching for your product. The results basically give you statistics on searches for this term. They can be broken down into category (by industry or market), timeframe or geographical location. You can also use the tool to view related search terms.
For example, plug in the term “child’s guitar” and you will see that the graph shows huge spikes in November and December for the last five years in a row. It’s a good bet that this is a popular Christmas gift.

Contrast this with the results for the phrase “guitar lessons” which shows a bumpy, but steady decline over the last five years along with flags from events in the news that contain the same phrase (for example, Lindsay Lohan apparently wanted to take guitar lessons from Slash this August).
If you assume that much of the time when people are interested in a certain type of product or service, they will “google it”, then you can use this tool as a gauge of interest in your product.
Have fun!