There are, but not as many as you might think. One consideration is the Bolt Circle Diameter (B. C. D.) You have to get that right so the holes to bolt on a new chainring will line up. See http://sheldonbrown.com/harris/chainrings.html. You also have to consider the capacity of the drivetrain to handle a range from high to low. From what I've heard, the capacity of most setups is wider than people think, especially if you have a triple setup.

Short answer: I had an idea for installing new chainrings to give myself a wider range. I just emailed Sheldon and asked if it would work. He replied the next day to say that it would be fine. Thank you Sheldon! (Everyone - patronize Harris Cyclery!)