Other Digital Modes

RTTY — old, 5-bit Baudot; very common. Often blasted long distances by high-powered transmitters.
WSJT — uses time-synched tones to send short messages huge distances at low power. Very slow; approximately 13 chars/minute.
Olivia — slow but interference-resistant multi-tone transmission mode.

WSJT is the only one I haven't got working so far due to an incompatibility under Ubuntu with my external sound card.

It really is remarkable; I've seen it make and confirm contacts with an antenna inside a building.

WSJT's strict time sync requirements can be a problem for Windows, but NTPD makes this simple on Linux.

RTTY — old, 5-bit Baudot; very common. Often blasted long distances by high-powered transmitters.