I've tried several of these. Fitday is good but it's more for logging in food than a complete package of fitness, healthy habits, etc. I also like Cronometer for the food log but not for anything else. I recently put in my stats and it suggested that I need to eat 800 calories to lose a pound a week! 800? I don't think so... (and I know it's not true from other daily calorie intake tools that I've looked at, which all give me a range closer to 1400-1500).
The one that I like the best is Sparkpeople. It has a lot of great support and the layout is pretty easy to use. It also has a blog, a place where you can make inspirational "stickies" that flash on your homepage to keep you motivated, and a tracker for other health habits (like drinking enough water or getting enough sleep).