A friend of mine is on a diet for science (they're taking part in an investigational weight-loss program through a local university), and so they have to count calories very carefully. So far, they've been using a handbook by "the CalorieKing", provided by the program, as well as internet searches of varying reliability.
When I've wanted to look up nutritional information -- say, after a fitness professional friend suggested that I increase my magnesium consumption -- my go-to resource has been the super-comprehensive but somewhat user-unfriendly USDA National Nutrient Database. Dig it. There are some 8,000 foods in the system, both organized by category and searchable by name. Look at the nutrition packed in that CSA kale we got this week! The only question I'm left with is, what's the percentage of my DRI (formerly known as Recommended Daily Allowance, or RDA) for all those nutrients? I have to cross-reference with another USDA website or a PDF file from the Institute of Medicine, which is too bad.
Google to the rescue, maybe. They've just announced that they're rolling out a nutrition data aggregator. Soon you'll be able to type "how much magnesium in kale" into Google search, and you'll get an honest-to-goodness Nutrition Facts label right there on your computer or handheld screen.
Neato burrito, so to speak.