There are a few apps on the Market that enable users to set up LED notifications and "LED Me Know" is the latest addition to the Market.

Developed by XDA forum member p0w4p0ty, LED me know enables users to choose LED colour and frequency for Gmail, SMS, missed calls, battery and disk space notifications.

Also, configurable is a notification for the notification toolbar with ringtone and vibration for your Gmail inbox.

Note that this is a beta version for the HTC Desire although since release, it has been reported that the application works on a variety of devices. In order to enable the app to work, stock LED notifications need to be disabled.

LED me know is available to download free from the Market.

For more information, take a look at the application thread.