Google Talk Application
IM : XMPP protocol (old name: Jabber)
Sound/Video : Jingle (XMPP extension)
Supported : iPhone, Android, MAC, Windows and Linux
Packet compression : –
Owner : Google, Inc.

Skype Application
IM : Skype protocol
Sound/Video : Skype protocol (Skype-to-Skype call)
Supported : iPhone, Android, MAC, Windows, Linux and Skyp over VoIP network (Digium and Asterisk)
Packet compression : Arithmetic coding algorithm
Owner : Microsoft, Inc.

Cisco Jabber Application
IM : XMPP protocol
Sound/Video : sip protocol (VoIP) H.264
Supported : iPhone, Android, MAC and Windows
Packet compression : –
Owner : Cisco Systems, Inc.