If you can get past the horrendous app icon -- or lack thereof -- the 99-cent Allergies iPhone app can be useful if you suffer from seasonal allergies. Tap the plain green app icon and you'll get the pollen count for your location. You can let the app use location services to find your current spot, or you can turn off autodetect to search manually by ZIP code or city name. (I found that the app didn't recognize many cities, but searching by ZIP code worked without a hitch.) Also in settings, you can choose from an insanely long list of colors for the background.
Allergies displays the current pollen count, which the app refuses to tell you anything about. On the developer's FAQ page, however, you'll discover it's on a 12-point scale and taken from Pollen.com. (Low is 0-2.4, Low-Medium is 2.5-4.8, Medium is 4.9-7.2, High-Medium is 7.3-9.6, and High is 9.7-12.0.)
Along with the pollen count, the app tells you which predominant pollens are in the air, such as grass, ragweed, and cenopod. Below the pollen number, the app shows you the current weather conditions. Tap on the pollen count to get a three-day pollen forecast.
In related news, the Smoggy iPhone app provides a similar service for air quality.