Here is a listing and links to popular Digital modes.
PSK31 or "Phase Shift Keying, 31 Baud" is a popular computer-soundcard-generated radioteletype mode, used primarily by amateur radio operators to conduct real-time keyboard-to-keyboard chat, most often using frequencies in the high frequency amateur radio bands (near-shortwave). - Wikipedia
The "Official" site can be found here.
"Automatic Packet Reporting System (APRS) is an amateur radio-based system for real time tactical digital communications of information of immediate value in the local area" - Wikipedia
The Home of APRS can be found here.
"Radioteletype (RTTY) is a telecommunications system consisting originally of two or more electromechanical teleprinters in different locations, later superseded by personal computers (PCs) running software to emulate teleprinters, connected by radio rather than a wired link". - Wikipedia
Slow-scan television (SSTV) is a picture transmission method, used mainly by amateur radio operators, to transmit and receive static pictures via radio in monochrome or color. - Wikipedia
Here is a site to get you started in Slow-scan television.