I have started experimenting with packet radio. As an aid to these, I created a very simple, very minimal utility for decoding the NZRI+HDLC encoding and reading AX.25 packets.
Packet-radio-minimal-receive is not a full TNC, but it is a useful diagnostic tool if you are writing your own packet radio software. It can display AX.25 frames in readable form, or dump them raw to stdout, or send them to another progrm over UDP.
Source, should compile under just about any *nix.