Diet Plans online

Number of Users 500, 000 since the site launched in 2003

Cost $5 per week

How It Works It's an interactive version of the popular book series published by Rodale, the parent company of Women's Health. Registered members have access to 12 weight-loss tools, including weekly meal plans, a virtual weight-loss buddy to help motivate them, and a staff of registered dietitians to answer questions.

What the Experts Think The interactive options are great. "Features like the online weight tracker and journaling encourage you to come back to the site every day, " Dr. Anderson says. That accountability is missing from the book and from some other online programs. And the online buddy is a great way to stay on track — if you're both motivated.

What We Think We love the fact that dietitians answer most posted questions in 12 to 24 hours and that you can call a dietitian with a specific query. The meal plan is a little dull: By the second day, the menu is already repeating some snacks and desserts (get used to something called Ricotta Crme). To vary it consult the site's database of more than 900 recipes.

Verdict The glut of special support features makes this site effective, as long as you're into the idea of low-carb dieting.

Grade: B+ (higher for South Beach fans)

DietWatch.com

Number of Users More than 1 million since the site launched in 1999

Cost $3 per week

How It Works Users register their weight-loss goals, then get one of four weekly meal plans: a low-calorie plan, a low-cholesterol plan, a reduced-carb plan, or a vegetarian plan. Or you can log everything you eat to make sure you're on track calorie-wise.

What the Experts Think They weren't impressed by these meal plans, which rely on supplements for nutrients like calcium. "You should be able to get the nutrients you need from the foods you eat, " Gidus says.

What We Think We liked getting weekly e-mails reminding us to weigh in — DietWatch is the only one of these sites to send them. But support and motivation aren't plentiful here. There's an "Ask the Dietician" feature, but we had to wait 3 to 4 days for a response.

Verdict Its nutritional advice isn't as sound or as structured as Weight Watchers; the fitness section isn't as personalized as eDiets; and the nutritionists aren't as quick as South Beach.

Grade: C

FitDay.com

Number of Users 1.8 million registered users (the site doesn't track how many are regular users).

Cost Free

How It Works This Web site isn't truly a diet at all. It's akin to the financial software Quicken, but instead of debts and income, you track calories burned and consumed. A variety of charts and graphs help you track your progress.

What the Experts Think Journaling your food choices is a great way to develop a strong sense of personal accountability, Gidus says. "You might think, 'Okay, I know the chocolate cake isn't the best choice, ' but when you log it you see how quickly the calories really add up."

What We Think The process of logging your meals and workouts is a bit tedious, and if you aren't already well versed in nutrition principles, you'll easily feel lost.

Verdict This site is best for self-starters who are comfortable enough with nutrition to make their own food choices. If you want a diet plan or any hand-holding, look elsewhere.