I read a wonderful article in Cycling Magazine last summer and it was all about sprints. I followed their advice and added sprints to my rides. Sprints are a great way to get a lot more bang out of a short ride. Their research showed (who knew they were doing all this research) that a half hour ride with four 30-second sprints with a 4-minute rest in between them was roughly equivilent to a two hour regular ride (I'm sorry I don't have the magazine so I can't give you the source of the research).
My experience with sprints is that doing even one sprint increased my fitness by quite a bit. It also taught my body to do fast, powerful accelerations whenever I needed them. This comes in particularly handy at intersections, especially when the light is yellow ;-)
Sprints are pretty powerful exercise. I've been doing them for almost a year now and I'm up to two in a ride. If you want to try one, find a flat or slightly uphill section of road without obstructions for at least a quarter mile. Using a watch or bike computer, ride like hell for 30 seconds shifting up as you need to. You get extra points if you can do this standing up on the pedals (I can't manage that yet). When you have finished your sprint, you should be pretty out of breath. Give yourself four minutes to recover, and if you're feeling it, do another one! I bet you are surprised by how much they increase your fitness.