Here are some useful links about Automatic Position Reporting System (APRS).

This is web site for Bob Bruninga WB4APR, the "father" of APRS:
http://web.ew.usna.edu/~bruninga/aprs.html

Northwest APRS is a very active group in the Pacific Northwest.
http://www.nwaprs.info/

And this is a "wiki" with lots of useful APRS info.
http://info.aprs.net/index.php?title=Main_Page

APRS Internet Server interfaces:

Find-u
http://www.findu.com/cgi-bin/find.cgi?call=N9HC*