There are a HANDFUL of things that it could be. We've seen CPS failures, clogged EGR valve, bad O2 sensors, bad air pump (180 mentioned that) and several other failures turn the light on.

The CPS is the Crankshaft Position sensor, a REAL common item to fail, sometimes it lights the light, sometimes it just fails and your car quits.
I've never had a failure, but I carry a spare CPS at all times, due to the problems others have had. They are hard to find and expensive locally, but you can order them online for as little as $50.