APRS Resources:

APRS Clients:

APRSISCE/32 for Windows: http://aprsisce.wikidot.com

YAAC (Yet Another APRS Client) for Windows: http://www.ka2ddo.org

PinPoint APRS for Windows: http://www.pinpointaprs.com

Xastir for Linux: http://www.xastir.org

PulseModem A for iOS: http://www.pulsemodem.com

APRSdroid for Android devices: On Google Play at http://play.google.com/store/apps/details?id=org.aprsdroid.app

SARTrack for Windows: http://www.sartrack.co.nz

APRSdos and APRS info: http://www.aprs.org

TNCs and Radios:

Kantronics KPC-3+ KPC-9612 KAM-XL: http://www.kantronics.com

Kenwood TM-D700A TM-D710A TM-D710GA TH-D72A TH-D74A radios: http://www.kenwood.com

Alinco DR-135TP DJ-MD5TGP radios: http://www.alinco.com

Yaesu FTM-100DR to 400DR FT1-DR to 3DR VX-8R radios: http://www.yaesu.com

AnyTone AT-D78UV radios: http://www.bridgecomsystems.com

Byonics MicroTrak series: http://www.byonics.com

Argent Tracker2 series radios: http://www.argentdata.com

QRP-Labs LightAPRS trackers: http://www.qrp-labs.com

Tracksoar balloon trackers: http://www.tracksoar.com

AVRT AVT510 tracker: http://www.avrtx.cn

Other Resources:

APRS: http://www.aprs.org 

APRS-IS on the internet: http://www.aprs-is.net 

APRS.fi to track any APRS station in the world: http://www.aprs.fi

APRSDirect.com to track any APRS station in the world: http://www.aprsdirect.com

Map.Findu.com to track any APRS station in the world: http://map.findu.com

NWAPRS: http://www.nwaprs.info

Backpacking with APRS: http://www.mthikes.com/backpacking-aprs/

Python APRS Module: http://www.github.com/ampledata/aprs

APRS Hackaday: http://www.hackaday.com/tag/aprs/

APRS Tier-2 Network: http://www.aprs2.net

Microsat Advanced Digipeater/i-Gate: http://www.microsat.com.pl

APRS Packet Reporting Systems: http://www.hamwaves.com/prs/en/

Winlink and APRS interoperability: http://www.winlink.org/APRSLink

APRS on Reddit: http://www.reddit.com/r/APRS/

Probably one of the best sites to learn APRS: http://www.wa8lmf.net/aprs/