Beagle Board has a series of LED lights. Those can be tested using the LED blink test program, which can be found on the link posted above.
It might be possible to create your own LED flashing pattern.
LED flashing pattern can also be used as a status indicator. One way is that it can be used to indicate whether a particular event has occurred such as boot completion. It can also be used to indicate status of a particular resource such as network connection, bluetooth connection, error detection.
One usage I can think of for this LED blink program is that, it enables an user to use a beagle board outdoors without having to use a laptop or a monitor to see whether a particular program is ready or not.
On the IRC channel, # beagle, IRC nicks GrizzlyAdams, Nader, were discussing about it. The thread can be found at http://www.beagleboard.org/irclogs/index.php?date=2009-11-19#T06:01:50