GNU Radio is a free software development toolkit that provides signal processing blocks to implement software-defined radios and signal-processing systems. It can be used with external RF hardware to create software-defined radios, or without hardware in a simulation-like environment. It is widely used in hobbyist, academic, and commercial environments to support both wireless communications research and real-world radio systems.
QRadioLink is a Linux software defined radio transceiver application using VOIP for inter-communication, built on top of GNU radio, which allows experimenting with software defined radio hardware using different digital and analog radio signals and a friendly user interface. Its primary purpose is educational, but it can also be customized for low power data communications on various frequency bands. It can also be used as an amateur radio SDR transceiver for demonstrating radio communications
Fldigi is a digital modem application for common amateur radio systems. It supports CW, PSK, MFSK, RTTY, Hell, DominoEX, Olivia, and Throb. It also helps calibrating a sound card to a time signal and perform frequency measurement tests. Flmsg is a messaging tool over digital radio, while Flarq supports ARQ file transfers.
Tomahawk is a music player and audio streaming service frontend for desktop usage. It manages songs, albums, playlists, meta info, social sharing. It supports various cloud services and commercial providers, such as Spotify, Beats, Google play, Soundcloud, LastFM, Hype Machine, Echo Nest, Rdio, Grooveshark, Billboard, OwnCloud, iTunes, Jamendo, Deezer, Ampache, Jabber, and Subsonic.
The WSJT project ("Weak Signal Communication, by K1JT") provides five programs (WSJT, WSJT-X, MAP65, WSPR, SimJT) designed for amateur radio communication. They include applications for meteor scatter, ionospheric scatter, EME ("moonbounce"), and QRP communication at HF.