To quote the ARRL,
Narrow Band Emergency Messaging System
"Narrow Band Emergency Messaging System (NBEMS) is an Open Source software suite that allows amateur radio operators to reliably send and receive data using nearly any computer (Windows, Mac, and Linux) and any analog radio without requiring a dedicated digital infrastructure or specialized modem hardware. NBEMS works on both VHF/UHF FM and on HF."
NBEMS is the standard digital emcomm package for Contra Costa County's Auxiliary Communications Service (ACS), Walnut Creek's CERT communications team and others.
The System uses Fldigi, the Fast Light Digital modem application, a free, open-source program which allows an ordinary computer's sound card to be used as a simple two-way data modem.
A Digital Practice Net is held in Contra Costa County every Tuesday evening at 8:00 PM using the W6CX 220/440 repeaters: 224.780 MHz, PL 77.0 & 441.325 MHz, + PL 100.0. Everyone with an interest in NBEMS is welcome to join in, ask questions and test your system.