Parent page: Internet and Networking
These are a few options:
Ekiga (previously known as Gnomeeting)
Qutecom (previously known as Wengophone)
Ring (previously known as SFLphone)
x-Lite (Non-free propriety)
They all seem to be in development. Ring seems to be the only softphone that supports PulseAudio out of the box. SFLphone and Twinklephone are reported to work properly with SIP implementations using Asterisk, but other SIP phones should be compatible with that as well.
Jitsi has rapid development, and a lot of features. However, it is Java based. (Which is an issue for some)
Ring and Jitsi are the most suitable softphones for companies, as they are the only softphones in the list to support attended transfer. (At the time of this edit) They also supports a wide range of codecs, as well as conference calls.