I have mine so dialed in I have
red = gmail + a tone of choice (GMail Notifier)
orange = SMS + vibrate + a tone of choice (handcent)
purple = a missed phone call + a tone of choice (missed call)
blue = calendar event + vibrate + a tone of choice (missed call)
green = Talk app chat (system)
only thing I wish, but not 100% needed, would be for Missed Call to be able to handle Verzions Visual Voice Mail (VVM). It would be nice if we could set an LED for that but, it is already kind oh handled by the missed call notification. I mean, with the missed call notification (purple for me) I can assume someone left a VM so I can see if VVM needs attention. but, it might be nice to be able to set a color to know for a FACT I have a VVM waiting. I have tried to contact the Missed Call dev, but he never ever responds.