voip-info.org
the defacto info source on everything Asterisk/VoIP

Digium
The driving factor behind Asterisk

Sangoma
The best PSTN cards for an Asterisk Installation

Elastix
The most versatile GUI for Asterisk currently available, includes Hylafax, Asternic CDR and Flash Operator Panel. Version 2.0 recently released.

Trixbox
One of the original Open Source GUI’s for Asterisk

FreePBX
The original GUI for Asterisk, one of the building blocks of the Open Source VoIP movement

PBX in a Flash
Another of the important building blocks of the Open Source PBX Movement

Xorcom
Makers of the only commcerially supported hot failover Asterisk system on the Market

Telcentris
Another Great SIP CLEC based in San Diego, CA

Asternic
Makers of the Flash Operator panel (FOP) and ccCdr graphic CDR reporting

PIKA Technologies
PIKA is another great telephony hardware Vendor

ICallGlobe
ICallGlobe is a directory of VoIP related service, providers etc.