Development of AX25 and other amateur radio related software is discussed on the linux-hams mailing list. To join, send mail to majordomo@vger.rutgers.edu 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/.