Google offers a free tool called Google Trends (originally called "Insights for Research") which is a great help with keyword research, necessary for optimizing your website and online presence. It can also help you figure out where your customers are located and indicate when they may be ready to buy.
Based on aggregated search data, Google Trends allows you to plug in terms, then look at patterns across a number of filters. You can narrow your search by Web, image, news or product. Select geographies from worldwide to city. Search as far back as 2004 to as recent as a specific date. Filters are also available by industry and interest. The tool allows you to compare results by search term, location and time ranges.
I gave it a try with skiing (sport) as a search term. The results were as expected, with interest peaking (yes, intentional) between January and April. Granted, this data is based on what people are searching for; there may be no direct correlation between those searches and actual purchases. But the resulting data, graphs and maps are pretty interesting. It just depends on how you interpret it.
In a recent search for keywords for a photography client, one that specializes in candid images, I thought it might be fun to compare "selfie" to the more common terms. The leap in the graph is a clear indicator of it's popularity. Whether it makes a good keyword for my client or not remains to be seen. That is, the common terms "candid photographer" or "profile pictures" are more directly correlated with her small business. Yet, I thought it couldn't hurt to try to get some spillover from a term that's just hot.