Oysters, while a known aphrodisiac and food associated with romance, are tricky. Depending on the season or location, certain varieties are better than others. Oysterpedia will teach you the difference between over 200 types of oysters, complete with photos and ratings. Plus, you can save information about oysters you've tried so you know exactly what to order if you're trying to impress.
Big oyster fans might want to spring for the $2 version of this app, which contains even more comprehensive information.