iChat is described as '(formerly iChat AV) is an AOL Instant Messenger (AIM), MobileMe, ICQ and XMPP client by Apple Inc. for their Mac OS X operating system. Using a Jabber-like protocol and Bonjour for user discovery, it also allows for LAN communication' and is an app in the Social & Communications category. There are more than 10 alternatives to iChat for a variety of platforms, including Windows, Android, iPhone, Online / Web-based and Mac. The best alternative is Jitsi, which is both free and Open Source. Other great apps like iChat are Hangouts (Free), Trillian (Freemium), Adium (Free, Open Source) and LINE (Freemium).
Jitsi (previously SIP Communicator) is an audio/video and chat communicator with full encryption that supports protocols such as SIP, XMPP/Jabber, ICQ/AIM, Windows Live, Yahoo!, GTalk/Hangouts extensions, as well as OTR, ZRTP, etc.
Trillian is an instant messaging app that works on phones, tablets, and desktops while keeping your chats neatly synchronized between all of them! We think conversation should be easily shared between all of your devices, not trapped as text messages on your phone.
LINE is a Japanese proprietary application for instant messaging on smartphones and PCs. Line users exchange text messages, graphics, video and audio media, make free VoIP calls, and hold free audio or video conferences.