MicroSIP!

Linphone is an internet phone or Voice Over IP phone (VoIP). With Linphone you can communicate freely with people over the internet, with voice, video, and text instant messaging. Linphone makes use of the SIP protocol, an open standard for internet telephony. You can use Linphone with any SIP VoIP operator.

Ekiga (formely known as GnomeMeeting) is an open source VoIP and video conferencing application for GNOME. Ekiga uses both the H.323 and SIP protocols. It supports many audio and video codecs, and is interoperable with other SIP compliant software and also with Microsoft NetMeeting.

csipsimple will allow native sip for android device.

What does it do:

Telephone is a VoIP program which allows you to make phone calls over the internet. It can be used to call regular phones via any appropriate SIP provider, including Google Voice via Gizmo5. If your office or home phone works via SIP, you can use that phone number on your Mac anywhere you have decent internet connection.

3CXPhone is a free VoIP phone available for Windows, iPhone or any Android based smart phone such as Google Nexus, Sony Xperia, Motorola Droid or Samsung Galaxy. Connect 3CXPhone to a VoIP provider or to a VoIP PBX (for example 3CX Phone System) to make calls to any VoIP, mobile or landline number.

Combining voice calls, video calls and Instant Messaging in a simple interface, CounterPath's X-Lite helps you seamlessly transition from a traditional phone environment into the world of Voice over IP.

Line2 is a VoIP phone service that lets you call & text from a separate phone number via WiFi or 3G/4G/LTE. At a small premium, Text & call ANYONE in the US/Canada, forwarding & more. Pick any phone number.

Lightweight portable SIP softphone with BLF and customizable keypad built with libre/librem/baresip BSD-licensed stack.

Paltalk is an instant messaging service that enables users to communicate via internet chat, voice and video chat. It offers chat rooms and the ability for users to create their own public virtual chat room.