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*
Substitute any callsign for N9HC. Use the wildcard (*) to get all of a callsigns SSIDs.

aprs.fi
http://aprs.fi/
Login in with your callsign

DB0ANF
http://www.db0anf.de/app/aprs

Here are links for APRS equipment manufacturers:

Kenwood TM-D710A
http://kenwoodusa.com/Communications/Amateur_Radio/Mobiles/TM-D710A

Kantronics KPC-3+
http://kantronics.com/products/kpc3.html

Byonics TinyTrak
http://byonics.com/tinytrak/

TNC-X
http://www.tnc-x.com/

Argent Data Open Tracker and Tracker2
http://www.argentdata.com/products/aprs.html

AGWPE -- Use your computer's sound card as a TNC. Requires a sound card interface such as West Mountain Radio Rigblaster or Tigertronics SignalLink.
http://www.sv2agw.com/ham/agwpe.htm

Links for APRS Software:

APRSPoint
http://www.aprspoint.com/

UI-View
http://www.ui-view.org/

XASTIR
http://www.xastir.org/