The TNC-Pi is a £22 ($40) KISS TNC board which is designed to connect to the GPIO port of the £25 ($35) Raspberry Pi computer board.
It provides a low cost means of using amateur radio AX.25 Packet Radio and APRS.
Nick Bown 2E0CGW has written a well illustrated review of the board. You can download the PDF from TNC-Pi by Nick Bown 2E0CGW
The TNC-Pi kit is available from http://tnc-x.com/TNCPi.htm