Google Trends (Very useful, for those who don't already use it)

It won't be useful for new startups since there isn't enough "volume" to create a trend.

It is, however, an interesting tool for observing on going patterns. Here is a trend I tried with "Web 2.0" and "Digg": http://google.com/trends?q=Web+2.0%2C+Digg&ctab=0&geo=all&date=all