Check out http://www.metricom.com/. Also the stanford "mosquitonet"
project. You can use them as "virtual modems" or you can put them in a
packet based "virtual ethernet" mode. 900Mhz, spread spectrum, rf
signalling rate at around 100khz, reliable throughput at 2.5Kbytes/sec
on a good line of sight, degrades smoothly in the presence of noise (I
had one radio in a machine room with unshielded cpu boards, that took
it down to about 400bytes/sec :-) It works better hanging from my
office window.
The metricom "ricochet" modem looks like a normal hayes modem with
*lots* of extra variables. The "packet" mode uses PPP-style quoting,
but lets multiple modems use a common channel (this is what the
Mosquitonet driver does.) http://mosquitonet.stanford.edu/mosquito.html
has details and the Linux driver, called STRIP (Starmode Radio IP.)