For the last month I have been beta-testing a site called "your.flowingdata.com" which uses the twitter API to create a simple data-tracker for personal information such as your weight and eating habits, your sleeping patterns, your feelings, and even your bowel movements. The site appears to now be out of beta-testing and open to all users.
I tried both "feelings" and "eating & weight." I didn't find much use for the "feelings" application, but I find it interesting to track what I eat and what I weigh as I try, again, to gain control of my personal mass. This is a simple application, but helpful as a regular reminder that what I put into my body has a connection to what I weigh and how I feel.
If you are interested in this sort of thing, and you are a twitterer, have a look and see what you think.