OwnerInfo widget for Android can be placed on your home screen (or lock screen if you use WidgetLocker application) and display information that you provide. It also lets the reader of this information call the phone number, send an SMS or e-mail with just one tap on the widget button.
Also, unlimited additional information can be displayed when one taps the widget. This allows you specify medical information if you have a disease that requires other people assistance in some cases (diabetes etc).
The source code is available on Google Code, together with issuer tracking capabilities.