NPR (New Packet Radio) is a custom radio protocol , designed to transport bidirectional IP trafic over 430MHz radio links (ham radio frequencies 420-450MHz). This protocol is optimized for "point to multipoint" topology, with the help of managed-TDMA. The net, effective Bitrate is up to 500kbps. The radio modem is cheap (~80$) and home-made, with a 433MHz ISM module inside. The modem is connected locally with Ethernet, therefore no specific software is needed on PC. The modem is easy to build and reproduce.
The entire project is open-source : hardware, software, protocol specification. See below for details