Yate is a next-generation telephony engine. While currently focused on Voice over Internet Protocol (VoIP) and PSTN, its power lies in its ability to be easily extended. It supports SIP, H.323, IAX, MGCP, Jingle, Jabber, E1, T1, analogic, robbed bit, ISDN PRI, BRI, and SS7. YateClient is an Instant Messenger and Voice application for Jabber/Jingle IM, SIP, H.323, and IAX VoIP protocols.
SafiWorkshop allows you to design, test, debug, and deploy advanced process flow applications (like IVRs) from a single, unified development environment. For database work, it includes a fully featured SQL query editor. SafiWorkshop also includes some Asterisk PBX specific features like an audio prompt editor.
PlugPBX is a prebuilt, ARM-based Debian system for end users to run Asterisk and FreePBX on the Marvell SheevaPlug low power platform. It includes Asterisk 1.6.1 with compiled DAHDI kernel mods, FreePBX 2.5, Apache2, MySQL, Samba, Munin, Webmin, Avahai, and OpenSSH. It is built on top of Debian Squeeze.
The Taridium ipbx is a complete software-based VoIP PBX system that replaces a traditional proprietary hardware PBX. It runs on Linux without the need for extra software licenses, and is based on the SIP standard. It supports many phones and is available for free in a 5 user version.