sparkpeople.com
I use it and love it. You can track food and nutritional information. It sets up a profile for you and lets you know how many calories you should eat and how much exercise you should do. There are a ton of forums and support groups so that you can talk to others who are in a similar situation.

Well I'm not sure why you posted this question in the Pet section, but if you are looking for weight loss sites for yourself, try www.nutrisystem.com, www.weightwatchers.com, www.jennycraig.com, www.medifast.com, www.healthychoice.com...a lot of them have free support sections that you can visit without actually buying their products or specifically following their diets. You can also try your family doctor for support. He or she can direct you to a nutritionist, who can further point you towards support groups.
If it is an overweight pet you have, you definitely need to see your vet. He or she can recommend a good nutrition/exercise plan for your pet to safely shed some pounds.

Well if your dog is over weight then maybe some one here can help. You posted this question on the Dog site! lol

Well I don't know any web site to go to for weight loss .