Digital Modes

This is the place for those with a particular interest in the digital modes of Amateur Radio.
APRS (useful comparison chart of APRS client software)
Packet / Winmor / Winlink2000 (send/receive email using your HF or VHF radio to the  or Winmor that ends up being delivered via the internet).
What do you need to get started over and above a normal station?
a) computer.
b) a way to interface your computer to the radio so that it can 'hear' the radio and 'talk' to the radio.
c) computer software.
Software Defined Radios - is a category of radio that uses digital signal processing techniques to decode/encode over the air signals. 
Check out for a list of receivers that you can use via the internet to listen to what they are able to hear.  While not strictly a 'digital mode', this is using digital techniques.