All about the digital mode D-Star from the JARL in Japan.  So far the main radio manufacture for D-Star is Icom.  Several other people have experimented enough with D-Star to also create homebrew D-Star Transceivers, Receivers, Dongles (which allow you to transmit on the D-Star network from your own PC, Laptop, etc.) and lots of software.

More information about D-Star can be found here.

EchoLink is a digital mode, but not in the sense that most people are used to.  It is a "user invisible" digital mode.  All data is sent as standard voice, but behind the scenes the voice is converted into digital information that is then sent through the internet.

You will find more details about the mode here.

Packet radio is a digital mode in which communications are sent in small chunks of data called packets.  It uses a standard packet protocol (X.25) that has been modified for use with Amateur Radio.  The modified form is called AX.25 and is great for sending data at 1200 bps and 9600 bps.  Although other speeds can be used as well.

Details about this mode can be found here.