Asterisk is an Open Source software VoIP PBX that is designed for home and small office use. It will run on most of the common operating systems out there.  Asterisk works great and there are now a lot of commercial companies using it as there telephone system.  While Asterisk works well it can be difficult to configure and setup, epecialy if you are new to the telephone world in general. 

trixbox is a complete OS, asterisk PBX, web front end, and a number of goodies all rolled up in an single ISO. If you are new to asterisk, then trixbox will help get you off the ground running.  There is nothing really new in trixbox but they have packaged a number of open source projects together on top of Linux CentOS, sprinkled in some glue, and come out with a full featured and easy to use asterisk PBX.