New applications of existing technology
One can find business ideas almost everywhere these days. A big category for ideas is new applications of existing technology.
For instance, the National Weather Service notes that it is able to see highway traffic patterns using its Doppler weather radar. I can see many interesting applications here. Scale down the radar, and focus on the highway rather than on the atmosphere, and you could have long range highway traffic flow monitoring without the expense of a video camera or sensor network.
Another simple technology applied differently is Apostrophree, which just received a $25 million investment for a spell checker dedicated to blog comments. While I don’t believe that there’s a $25 million company there, I always enjoy creative entrepreneurship.