I have used a quite few different programs (software) that do just that. One of the best I have used is the food processor from Esha Research
http://www.esharesearch.com/ .. It's pretty spendy. I've used a few PDA based programs as well. It would be cool if there was an online version that let you compare your results to other people as well as collaborate on goals etc. You could even throw in a PDA synchronization tool. Take the PDA to the gym with you and use it as you workout then synchronize the PDA with the online site. Could be a very dynamic fun site with endless features and ideas. Unfortunately most people can barely get in the gym, so all the added steps may be difficult for people to complete.
-Tony