From time to time, I’ve used FitDay to lose a little weight. It does an okay job of enabling you to track the calories you eat and burn. The Web app doesn’t seem to have changed in years, though, and the food choices are extremely American (lots of junk food, no tabouleh).
More importantly, FitDay lacks a strong user community, and doesn’t enable users to share diet-related data (recipes, before and after photos, weekly diet plans, etc). Given the success of the social aspects of Weight Watchers, and a general trend toward sharing personal information online, I think the world needs a Flickr for dieting. It needs to be fun and easy to use, have tagging and RSS feeds and foster strong connections between users.
I was mentioning this to Kris recently, and he pointed out that the 2005 business plan is “Flickr for X“.