Here are some resources for information about nutrients in food.
Search the USDA National Nutrient Database
The USDA has nutrient data for a large number of foods. You can even download their entire data base if you want. They also have convenient Nutrient Lists sorted alphabetically or by nutrient content.
Danish National Food Institute
A much smaller but complimentary nutrient data base with measurements for European foods.
Linus Pauling Institute
Excellent detailed information about nutrients.
Worlds Healthiest Foods
More good information about nutrients in food and a few anti-nutrients. Just ignore their fat phobia and love of soy :)
They offer nutrient data from the USDA data base in a convenient form. Be sure to ignore their misguided conventional dietary advice. They require registration to get full access to their most useful features, but it's free and so far I have not had any spam from giving them my e-mail address. To get summaries of the nutrients in the food you eat, first search for each food item and add it to your "Pantry" by clicking on the "Add to Pantry" button at the top of the food nutrient information. Then go to the "Pantry" (menu selection) and enter the food amounts for a meal or a day's worth of food and click on the "Total +" button at the bottom. All the nutrients are quickly tallied for the foods and amounts you have selected. If you enter or change amounts, it keeps accumulating the results until you click the "Total 0" button at the bottom of the "Pantry" to reset to zero. You can also save "Recipes" for foods or meals you make with multiple items. Overall it's a very powerful, fast, and relatively easy to use system. Too bad they don't have better dietary advice.
Offers free online tools to help you monitor your daily nutrition and exercise to meet health goals. I haven't tried it yet but it looks useful. I did look at the tutorial and it looks like FitDay is geared towards weight loss.
Dietary Nutrition Calculator
This is my Excel workbook that allows you to estimate your daily intake of nutrients from the foods you eat. Click the link above for more information.