The Linux AX25 HOWTO contains detailed information on setting up packet radio under Linux. The Linux HAM HOWTO describes other software packages of interest to radio amateurs. Both are included in most Linux distributions and are available from major Linux archive sites.
Development of AX25 and other amateur radio related software is discussed on the linux-hams mailing list. To join, send mail to firstname.lastname@example.org with the text “subscribe linux-hams” as the message body.
Three good starting points on the World-Wide Web for learning about amateur radio and packet are:
A software repository of source code, documentation, and binaries of most ham-radio related packages can be found at http://ftp.ucsd.edu/pub/hamradio/.