What does it mean when your car engine starts revving high when in Neutral?

I drive a stick shift and sometimes when I am shifting or when I start the car and let it sit in Neutral the rpms increase with no gas added. For instance, last night I started the engine and let it sit for a minute to let it warm up and the rpms jumped up pretty high (between 2 and 3 on the meter) and steadily increased to almost 4 on the meter. What can possibly be causing this?