Gajim

The goal of Gajim is to provide a full featured and easy to use Jabber client. Gajim works nicely with GNOME, but does not require it to run. It is released under the GNU General Public License.

FEATURES:

• Tabbed chat window
• Group chat support (with MUC protocol)
• Emoticons, Avatars, File transfer, Room Bookmarks
• Metacontacts Support
• Trayicon, Speller, extented chat history functionalities
• TLS and GPG support (with SSL legacy support)
• Transport Registration support
• Service Discovery including Nodes
• Wikipedia, dictionary and search engine lookup
• Multiple accounts support
• DBus Capabilities. Read more information
• XML Console

Homepage: http://www.gajim.org

Pidgin

Pidgin is an instant messaging program for Windows, Linux, BSD, and other Unixes. You can talk to your friends using AIM, ICQ, Jabber/XMPP, MSN Messenger, Yahoo!, Bonjour, Gadu-Gadu, IRC, Novell GroupWise Messenger, QQ, Lotus Sametime, SILC, SIMPLE, MySpaceIM, and Zephyr.

Pidgin can log in to multiple accounts on multiple IM networks simultaneously. This means that you can be chatting with friends on AIM, talking to a friend on Yahoo Messenger, and sitting in an IRC channel all at the same time.

Homepage: http://www.pidgin.im

Miranda

Miranda IM is a multi-protocol instant messaging client for Windows. Very light on system resources and extremely fast, Miranda IM requires no installation and can be made to fit on a single floppy disk or USB drive. Featuring a powerful plugin-based framework and boasting over 350 plugins, Miranda IM is one of the most flexible and customisable messaging clients on the planet.

Homepage: http://www.miranda-im.org

Spark

Spark is an Open Source, cross-platform IM client optimized for businesses and organizations. It features built-in support for group chat, telephony integration, and strong security. It also offers a great end-user experience with features like in-line spell checking, group chat room bookmarks, and tabbed conversations.

Homepage: http://www.igniterealtime.org

Jabbin

Jabbin is an Open Source instant messaging program that allows free PC to PC calls using the VoIP system over the Jabber network. The new Release 2.0 use the libjingle protocol.

Homepage: http://sourceforge.net/projects/jabbin/

Ekiga

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.

Homepage: http://www.ekiga.org

AIM

AIM helps you manage your busy social life. Challenge them to fierce online games, make your stories come to life with photo sharing and much more. Plus, your AIM buddy profile is a great place to leave taunting messages for your friends. Sometimes you have to hear it to believe it. Use Voice Chat to go from typing to talking with anyone on your Buddy List window. Talk all you want because it's free*. *No equipment installation required. All that's needed is a computer with speakers and a microphone. A headset or USB phone and high-speed connection are recommended, but optional. Look your buddies in the eye while you chat with them. Video chat lets you switch from an Instant Message chat to a high-quality video conference.

Homepage: http://www.aim.com

ICQ

ICQ was the first instant messenger client available - pioneered by Mirabilis in 1996. For many years ICQ was the most popular client on the market, but lately it has been surpassed by MSN Messenger, AOL Instant Messenger (AIM) and others. However, ICQ still has many advanced features and has been the first to introduce a number of cutting edge functionalities - such as sending SMSes. Apart from standard chat ICQ also supports video and voice (VoIP) chat. Skinning is also an important part of ICQ allowing people to change the look and feel for their ICQ client. ICQ is available in virtually any language.

Homepage: http://www.icq.com

Agent Mail.Ru

Freeware IM messenger with audio and video chat.

Homepage: http://agent.mail.ru

ooVoo

ooVoo is the next evolution in online communication — a remarkably easy way to have a face-to-face video conversation with friends, family or colleagues, no matter where they are in the world. ooVoo is remarkably easy to use: easy to download, easy to install, and best of all: it's FREE!

Homepage: http://www.oovoo.com

Windows Live Messenger

Windows Live Messenger is the next-generation MSN Messenger. It has everything you already love about Messenger—your contact list, emoticons, and instant access to your friends via text, voice, and video—plus new ways to connect and share photos and documents effortlessly. As always, it's free to download Messenger and use most of its features.

Homepage: http://get.live.com/messenger/overview

Sightspeed

SightSpeed Video Chat lets you connect with all your friends and family easily. Make video calls, phone calls, and send video mails and text messages to everyone in your network, anywhere in the world.

Homepage: http://www.sightspeed.com/personal

Wengo

OpenWengo is a community of enthusiasts and developers, creating free software products related to communication over IP. The flagship product of the OpenWengo 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.

Homepage: http://www.openwengo.org

Express Talk

The Express Talk softphone works like a telephone to let you make calls through your computer. You can call anyone on the internet who has installed it (or any other SIP softphone). Calls computer to computer are always free. You can also call ordinary 'real' telephone numbers anywhere in the world if you sign with a VoIP gateway service company. See here for some recommended telephone gateway companies.

Homepage: http://www.nch.com.au/talk/

Gizmo

The Gizmo project offers free calls from a computer to the landline/mobile phones of other Gizmo users, but with restrictions that aren't fully disclosed. As a result, accounts can switch between a qualified and unqualified status without notice.

Homepage: http://www.gizmoproject.com

Google Talk

Google Talk is a Windows application for Voice over IP and instant messaging, offered by Google. The first beta version of the program was released on August 24, 2005. Instant messaging between the Google Talk servers and its clients uses an open protocol, XMPP, allowing users of other XMPP clients to communicate with Google Talk users. VoIP in Google Talk is based around the Jingle protocol. The technology used within the Google server network however is not publicly known.

Homepage: http://www.google.com/talk

Skype

Skype allows users to make telephone calls from their computer to other Skype users free of charge, or to landlines and cell phones for a fee. Additional features include instant messaging, file transfer, short message service, video conferencing and its ability to circumvent firewalls.

Homepage: http://www.skype.com