CellConMon is a simple mobile phone network connectivity monitor that tells you when you lose and re-gain connectivity with the mobile phone network (GSM/3G). The application just uses the notification bar and the LEDs to notify you about connectivity changes. There is no application or GUI.
I created the tool as a countermeasure for our SMS-based Denial-of-Service attack where we can knock Android phones of the network. The tool will notify (notification bar massage and RED blinking LED) you if you lose connectivity. I also found it to be useful in normal (not being attacked) situations where you lose connectivity due to being underground or whatever.
The tool is in the Market but can also be downloaded from my Android page.