I just found Elastix at Freshmeat, it looks pretty cool. Its an open-source communications server, combining email, instant messaging, fax, and voice over internet protocol (VOIP). They mention a similarity to Trixbox, or better yet, that Trixbox is borrowing some of their code, so I'll include some information about what that's all about.
Trixbox is a "telephony application platform" based on Asterisk. From what I can tell, it is more focused on voice over internet protocol than Elastix.
Both of these projects are open source, so it is perfectly legitimate for each of them to use each other's code as they please. That's what makes open source software so great!
Elastix is a more complete solution for VoIP.