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ICQ (or 'I Seek You') is an instant messaging service that allows you to find friends, colleagues and people with similar interests.

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

Digsby is a multiprotocol IM client that lets you chat with all your friends on AIM, MSN, Yahoo, ICQ, Google Talk, and Jabber with one simple to manage buddy list.

Adium is an instant messaging application for Mac OS X that can connect to AIM, MSN, Jabber, Yahoo.

ooVoo is a tool that allows you to add up to 6 participants to your video call, video call in high resolution, record video and audio calls, or share your desktop.

eM Client is an application which integrates Email, Calendar, Instant Messaging and offers users the ability to adjust the interface to meet the unique needs of your organization.

QuteCom (formerly WengoPhone) is a multi-platform VoIP client. The GUI is Qt-based, and the Video-over-IP engine is based on the eXosip, oSIP, oRTP, ffmpeg, and libgaim projects. The eXosip module is extended by a phApi module, which implements a high-level, easy-to-use call control API. It supports PC-to-PC voice, video, and chat. One can use a standard SIP service provider such as Wengo to be assigned an incoming number, make calls to PSTN/cell phones, get voice mail, and more...