Originally launched last year, Google has provided more functionality to their nutritional database, and you can get the information that you need without leaving the search engine. Simply do a search to compare two different foods, or use "vs" instead. (You need the double quote marks). Here's a comparison of crisps and apples:
The pulldown option is worth noting, since it gives a huge number of different comparisons, such as vitamins, sugars, proteins and fats. Unfortunately it will only compare two different things at once, but it's still a valuable tool if you're into nutrition.