Elastix is a linux distribution / a package / a bundle based on CentOS 5.x, Asterisk, FreePBX and several other utilities. In addition to the standard FreePBX features, some special features include the built-in drivers of most of the telephony cards, easyily configurable phones, Asterisk2Billing, Conferencing, Conference-Bridges, GUI for Network Setup, Backup and Restore, updates, SugarCRM, VTiger CRM, mail client and a lot more.
Elastix is FREE. Thats the best quality of this system compared to it predecessor Trixbox who have commercial and freeware editions, which really sucks. But its the choice of people using it, Elastix is new compared to something in the market for years. So the users are very limited, but growing at a very fast speed.