What FLOSS skype alternatives are available? [closed]

Given Skype's slow development speed for the GNU/Linux version and its recent acquisition by Microsoft, I am looking for alternative softwares.

What FLOSS (free/libre/open-source software) Skype alternatives are available?

SIP Clients

These all are SIP clients which means they all can be used to call each other and they just need a SIP address (like your mobile number). You can get a SIP address for free from many providers (Ekiga, jit.si, iptel), some of which (sip2sip.info, ippi.com, localphone.com, nonoh.net) also have PSTN gateways into the normal phone network which allows you to make and receive regular calls from/to your SIP client (for a fee). For various reasons some clients may not work with some providers (for example, for Nonoh and Iptel, Blink works but Jitsi, Ekiga, and Linphone don't).

Ekiga (available in the software center)

Here is First use documentation for ekiga .

Here is documentation on how to use the PC to Phone feature in ekiga .

Ekiga (formely known as GnomeMeeting) is an open source SoftPhone, Video Conferencing and Instant Messenger application over the Internet.

Ring (available in the software center)

Ring is free software for universal communication which respects freedoms and privacy of its users. It supports SIP accounts. Ring is available under the GNU GPL license, version 3.

Twinkle (available in the software center)

Twinkle is a softphone for your voice over IP and instant messaging communcations using the SIP protocol. You can use it for direct IP phone to IP phone communication or in a network using a SIP proxy to route your calls and messages.

Yate VoIP (available in the software center)

YATE is a telephony engine aimed at creating a telephony server that performs well enough to deal with PBX requirements and also flexible enough for complex Gateway and IVR solutions.

YateClient uses the same telephony code to produce a software desktop VoIP (SIP, H.323, IAX2) phone. yate-qt4 uses this to provide such a telephony

QuteCom (available in the software center)

QuteCom is a community of enthusiasts and developers, creating free software products related to communication over IP. The flagship product of the QuteCom project is a softphone which allows you to make free PC to PC video and voice calls, and to integrate all your IM contacts in one place.

Linphone (available in the software center)

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, including our free SIP audio/video service

Jitsi

Jitsi - Open Source Video Calls and Chat Secure video calls, conferencing, chat, desktop sharing, file transfer, support for your favorite OS, and IM network. All this, and more, in Jitsi - the most complete and advanced open source communicator. Implemented in Java.

Blink

A state of the art, easy to use SIP client Available for Mac, Windows and Linux. Comprehensive controls over the details of the protocol. Blink is the end-result of cumulated knowledge built over many years by its developers who worked on the development of popular open-source SIP infrastructure software and attended relevant IETF and SIPIT events where SIP protocol has been developed and tested 18. Implemented using Qt.

You can use Empathy (pre-installed in Ubuntu) instead of Skype.

For users of Windows, OS/X, you can ask them to install Google Voice and Video chat, http://www.google.com/chat/video

Empathy and Google Voice and Video chat are compatible with each other, so you can have voice and video chats!

There is an option for VoIP (SIP) support in Empathy. You install the telepathy-sofiasip package to add SIP support. It might not be excellent yet, however you can get the work done.

In addition, with Empathy you have many additional options (not found in basic Skype), including

• you have the option for remote desktop to your IM contacts (pre-installed)
• you can SSH to the computer of your contacts (requires package to install)
• you can collaborate with your contacts in writing a document with gedit (see gedit-collaborate).