A database is a system intended to organize, store, and retrieve large amounts of data easily. All the databases linked here are of course related to weight loss.

USDA National Nutrient Database for Standard Reference

This is "the big one". If you have ever seen books listing nutrient information for thousands of foods, this is where that information came from. This database is based upon hundreds of thousands of careful measurements performed at the United States Department of Agriculture's 'Agricultural Research Service, Food Nutrient Laboratory'. 

This is the largest database of glycemic indices in the world. It is maintained by the Glycemic Index Foundation and uses data obtained by Sydney University's Glycemic Index Research Service (in Australia).