Below find some of the best VoIP products, services and companies.  These are products and companies we routinely use and rely on for business telephone systems:

Digium
The driving factor behind Asterisk, now a subsidiary of Sangoma

Sangoma
Carrier grade PSTN cards, DSP and Gateways for Asterisk Installations

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

Asterisk Project Wiki
The Asterisk Project Wiki – All things Asterisk!

Xorcom
Manufacturers of the only commercially supported hot failover Asterisk system on the Market

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

Poly (formerly Polycom & Plantronics)
Some of the best performing and well constructed VoIP Phones and devices on the market

SuiteCRM
A premier Open Source CRM solution, based on SugarCRM

Dokuwiki
A simple to use and highly versatile Open Source Wiki that doesn’t require a database

PBX in a Flash
One of the important (former) building blocks of the Open Source PBX Movement

Grandstream
Grandstream offers a comprehensive line of VoIP phones with a full feature set

Algo
Canadian manufacturer of specialty VoIP endpoints, such as door phones/intercoms, paging adapters, strobe lights, and push buttons

Ribbon, formerly Edgewater Networks
Manufacturer of Carrier grade VoIP gateways seamless convergence

Issabel
Issabel is an open-source Unified Communications PBX platform based on Elastix (and FreePBX) that features Call Center management and reporting, Email integration, Fax and chat/instant messaging server capabilities as well as contact and celdnar management

Yealink
Yealink manufactures VoIP phones that offer a great feature set at a good price point

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