Webapp a Calorie Counter searches the USDA food nutrition database and provides instant nutritional information for thousands of generic or prepared food products. It's a lot more exhaustive than previously mentioned Calorie Calculator and the information is a lot more comprehensive. See what 200 calorie and 300 and 400 calorie meals look like, or follow other reader recommended weight-loss tips.
AU - Australian site Calorie King does a similar job. If you're using it as a food tracker, it has a lot of local supermarket brand foods preloaded so it can give you the exact calorie/kJ count for them.