There are a variety of softphones available for Linux, Windows and Mac OS.

Many users will get high quality audio by connecting a headset to the 3.5mm audio socket on their computer. On many modern laptops, the 3.5mm connector is a TRRS socket that integrates the microphone and stereo headphones into a single connection, as used on many popular mobile phones. However, using headsets with a USB connector yields better results in some cases.

The Jitsi softphone is developed in Java and runs on Linux, Windows and Mac OS. It supports both SIP and XMPP.

There is a detailed guide to setup up Jitsi with client certificates in the reSIProcate wiki.

The GNOME desktop on many common Linux distributions includes the Empathy softphone, supporting IM, voice and video over SIP or XMPP.

Various other Linux softphones are available, including LinPhone and Ring (formerly SFLphone).

There are a vast array of Linux messaging applications for XMPP, not necessarily having support for voice or video. Popular choices include Pidgin and Psi.

One of the most popular proprietary softphones is Bria from Counterpath.