Voip-info.org
The latest and best freeware/open-sourced Voip software for all platforms can be found at voip-info.org. The site contains open-sourced software for most platforms that includes; Windows, Linux and Mac OS X. Everything a developer needs to create, implement and test Voip capable systems can be found there along with tons of other free and useful stuff. This is a wiki-site that is considered to be the premiere place to go for developer tools and other stuff that has to do with Voip. Check out the site for more details and a more information about the available software. The site also accepts contributions to the archive of tools already there for others to use and benefit from.