Free Addons

Windows Server Failover Script
VoIP Quality Monitoring Tool Webservice
OCS/Lync to SIP Phone Integration
Free Music on Hold
Grade Your Broadband

Free Quality SIP Client - Blink (Windows, Linux, Mac, Mobile?)
Free SIP Phone System - snom ONE Phone System (Windows, Linux, Mac)
Free SIP Phone System - 3CX Phone System

Do you know more SIP stuff that I should list here? I'd be glad to hear from you so comment below.