Google Trends is a nifty tool from Google Labs. While it’s not perfect (and shouldn’t be used for any serious analysis), it does allow you to plot a rough approximate of the “zeitgeist” of search term (or terms).
For example, let’s look at the decline of the some of the bigger spyware/adware companies: 180Solutions, Direct Revenue and Claria:
You can see they are on a decline.
Or, let’s look at the overall trend in the search term “spyware”:
You can hit the site at google.com/trends.