Fldigi (short for Fast light digital)[4] is a free and open-source program which allows an ordinary computer's sound card to be used as a simple two-way data modem.

Such communications are normally done on the shortwave amateur radio bands in modes such as PSK31, MFSK, RTTY, Olivia, and CW (Morse code).

Flamp implements the Amateur Multicast Protocol by Dave Freese, W1HKJ[23] and is a tool for connectionless transferring of files to multiple users simultaneously without requiring any existing infrastructure.

[29] FLLog is a logging software which keeps track of conversations between amateur radio operators in a database format known as ADIF.

[32] This is a telnet client to remote DX cluster servers, which is a real-time reporting of stations heard transmitting, and their frequencies.

[34] FLRig is a component of the FLDigi suite of applications that enables computer aided control of various radios using a serial or USB connection.

[36] Currently 7 sound card-digital-modem programs support this standard: RSID operates by sending a short burst of a specific modulation before the data signal, which can be used to automatically identify over 272 digital modes.

For more complex uses or file transfer external programs can be attached to the internal TCP/UDP ports 7322 (ARQ), 7342 (KISS), and 7362 (XML-RPC).

Programs which have a history of use with Fldigi as the underlying modem include: The software is also utilized by some organizations for both routine and disaster/emergency relief services.

The Fldigi suite of programs has become popular within the U.S. Army and U.S. Air Force Military Auxiliary Radio System.

[56][57] Fldigi is being used in-testing as part of the Department of Homeland Security Shares program, which utilizes "existing HF radio resources of government, critical infrastructure, and disaster response organizations to coordinate and transmit emergency messages"[58][59] Fldigi is used as the underlying modem for the PSKmail project.