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Dire Wolf is a software "soundcard" AX.25 packet modem/TNC and APRS encoder/decoder. It can be used stand-alone to observe APRS traffic, as a tracker, digipeater, APRStt gateway, or Internet Gateway (IGate).
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With tncattach you can attach any KISS-compatible TNC as a fully ethernet-compatible network interface in Linux. It also supports more "lightweight" tunnel interfaces, for point-to-point links (where you don't need the overhead of ethernet).
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Pat is a cross platform Winlink client.
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Paracon is a packet radio terminal for Linux, Mac and Windows. It is focused on simplicity and ease of use, and incorporates the core functionality that most packet users need without trying to include all of the bells and whistles that few would use.
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PakCatt is a modern packet radio framework built on top of KISS and AX25. It's name hails from the much loved PAKRATT TNC modems used in the heyday of packet radio. It's highly extensible and able to integrate into other modern web services.
Not actually Linux but seems interesting
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WoAD brings the functionality of Winlink system to your Android device. Using WoAD you can send and receive e-mail messages through a Winlink Common Message Server (CMS), using either a VHF/UHF packet or internet connection. WoAD also supports peer-to-peer (P2P) connections to other Winlink P2P-capable clients, again through a packet or internet connection.