FitDay is perfect if you want a Web based solution and you don't mind the fact that the raw data is kept someplace else. Since I'm a geek and wanted to add my diet log to my own Web site I use
EatWatch for Palm OS to keep track of my weight and export the charts to my
home page. Also, since the raw data is local to me, I can import it into other software such as diet trackers, weight training logs, etc. Here's an example of an EatWatch moving average chart with trend analysis...it gives you a better idea of your actual weight loss progress even if your weight bounces all over the place (as opposed to FitDay's blocky chart):