Imagine you are on a military recon mission. You've found where the bad guys live. You know where the hidden arms cache is. You know where the locals must be protected. You need to tag these locations in a way that is unnoticed by the enemy but can be easily seen and read by the good guys.
You take out a device about the size of a cell phone or PDA, and enter a message up to 60 characters long. The device translates the message into a set of modulated chemistry (yes, chemistry), embeds them on piece of paper, and ejects the paper. You take the piece of paper (covered with an adhesive backing), stick it on an unobtrusive spot, and move on.
Chemical reactions on the paper cause it to glow in a narrow infrared band visible only to special equipment. The chemical reactions are designed to modulate the glow so as to transmit the encoded message. This chemically powered optical transmitter continues to broadcast the message for 100 hours, so when the Good Guys arrive, they know exactly where to go and where to avoid.
A Request For Proposals to build such a device has just been let by DARPA. You can read all about it here.