+1 on the free app. I have a $35 computer that is 2.4ghz ( i think) that tells trip odom, odom, speed, avg speed, stopwatch, and clock. I also have a phone app running called runkeeper which works with my fitness pal for dieting. The only downside to the phone app if it's all you have is you don't get easy instant speed readout and more an average speed at the end of your ride. Also, you must remove your phone and pause the function if you stop otherwise your average is skewed due to the period of inactivity.

I enjoy the total miles odometer as I am on a strict budget. With the odometer I "dream" of a new piece of gear to buy and then put a mileage goal on it. Not only do I have to save the money but I have to earn it with some sweat to "prove" it's worth to me.