From the City of Frederick:
The City of Frederick and Parkmobile USA, Inc. announced today that a revolutionary new service for parking transactions has been implemented in The City of Frederick. Customers can now pay for parking with their cell phone using Parkmobile’s native mobile applications for the iPhone, Android, Windows 7, and Blackberry smartphones.
Here is the link to download the app. I’d create your account on the Parkmobile site though, instead of through the app since you have to enter a credit card number.