Linphone!

Empathy is a messaging program which supports text, voice, video chat, and file transfers over many different protocols. You can tell it about your accounts on all those services and do all your chatting within one application.

Conversations is a privacy focused, open source XMPP Texting [Jabber] client for Android 4.0+ smart phones with wife-approved easy encryption [OTR, OMEMO, or GPG].

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.

Open source Jabber (XMPP) client with multi-account support, clean and simple interface. Being both free (as in freedom!) and ad-free, Xabber is designed to be the best Jabber client for Android .

csipsimple will allow native sip for android device.

Nimbuzz offers limited availablility free calls and limited availability free text messaging for users via 3G and WiFi. Calls to other Nimbuzz users are free. PAY to call mobile or landline phones for as little as $0.02/minute in bulk with NimbuzzOut. Free SIP to SIP calls.

What does it do:

Coccinella is a free and open-source cross-platform instant messaging client with a built-in whiteboard for improved collaboration with other people.

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.