Yahoo! Messenger is a popular advertisement-supported instant messaging client and protocol provided by Yahoo!. Yahoo! Messenger is provided free of charge and can be downloaded and used with a generic "Yahoo! ID" which also allows access to other Yahoo! services, such as Yahoo! Mail, where users can be automatically notified when they receive new email. Yahoo! offers PC to PC telephone, file transfers, webcam hosting, text messaging service, and chat rooms in various categories.

It’s features include Photo Sharing, LAUNCHcast Radio, Yahoo! Games, IMVironments, Yahoo! Calendar, Contact Search Bar, Find, Add, Share Friends, Yahoo! Search, File Transfer, Address Book, Stealth Settings, Plug-ins - New!, Sound effects - New!, Audibles, Emoticons, Yahoo! Avatars, Display Images, Skins, Chat Rooms, Make Calls From Your PC, PC-to, Phone Calls* - New! Phone-to-PC Calls* - New!, Connect on Yahoo! 360° - New!, Take, Yahoo! with You, Webcam and Conference

Product Details
Developer: Yahoo!
Latest release: 8.1.0.402 (Windows)/3.0b1 (Mac) / June 15, 2007
OS: Windows 98, Windows Me, Windows 2000, Windows XP, Windows Server 2003, Windows Vista, Mac OS 8, Mac OS 9, Mac OS X, Unix
Genre: VoIP/Instant messaging client
License: Proprietary freeware
Website: messenger.yahoo.com

Google Talk

Google Talk is Google's instant communications service. Google Talk offers you different ways to integrate real time communications into your online experience.  The Google Talk Gadget lets you see who's online and send instant messages from your Google Personalized Homepage, without installing any software.  You can also add the same instant messaging capabilities to your own webpage or blog by cutting and pasting a single line of code.  And by downloading the Google Talk Client, you can make voice calls and transfer files to your friends.

Google Talk Gadget:
Google Talk on your Google Personalized Homepage, Add Talk to your own webpage
Preview YouTube videos, Preview Picasa Web Albums, Archive your chats in Gmail, No software to download (but you need Flash Player 8.0 or above), Cost: Free

Google Talk Client:
Quick Download, Make free PC-to-PC voice calls, Send and receive voicemail, Transfer unlimited files and folders, Receive chat and email notifications, Archive your chats in Gmail , Cost: Free 

Product Details

Maintainer: Google
Stable release: 1.0.0.104  (January 1, 2007) [+/-]
OS: Windows 2000, XP, Server 2003, Vista
Use: VoIP/Instant messaging client
License: Proprietary freeware
Website: google.com/talk

MSN Messenger
MSN Messenger is a freeware instant messaging client that was developed and distributed by Microsoft between 1999 and 2005 for computers running the Microsoft Windows operating system, and aimed towards home users. Amongst its users it is often referred to as "MSN," though in Latin America and Iberian Peninsula it is known as "el messenger" or "Messenger"(and often misspelled "El Mazinger" due the popularity of the Anime "Mazinger Z") as other instant messaging clients are less popular there.

Product Details
Developer: Microsoft
Latest release: 7.5 / August 23, 2005
OS: Microsoft Windows
Genre: Instant messaging client
License: Freeware
Website: http://messenger.msn.com

ICQ
ICQ is an instant messaging computer program, owned by Time Warner's AOL subsidiary. ICQ was first developed by the Israeli company Mirabilis. The first version of the program was released in November 1996. The name ICQ is an oronym on the phrase "I seek you".

Product Details
Developer: Mirabilis, AOL
Latest release: 6.0 / April 17, 2007
OS: Windows, Macintosh, mobile phones
Genre: Instant messaging client
License: Adware
Website: ICQ

Rediff Bol
Rediff.com India, NASDAQ: REDF is a news, information, entertainment, and shopping portal. It was founded in 1996 and is headquartered in Mumbai, India with offices in New Delhi and New York, USA. Rediff Bol is an instant messenger with support for Hindi language

AOL Instant Messenger
AOL Instant Messenger (AIM) is an advertisement-supported proprietary freeware instant messaging and presence computer program which uses the OSCAR instant messaging protocol and the TOC protocol. It was released by AOL in May of 1997.

Product Details
Maintainer: AOL LLC
Stable release: 6.1.41.2  (May 3, 2007) [+/-]
Preview release: 6.2.14.2 [+/-]
OS: Windows (older versions for Mac and Linux)
Use: Instant messaging client
License: Proprietary
Website: www.aim.com/

Meebo
Meebo is an in-browser instant messaging program, like AIM Express, which supports multiple IM services, including Yahoo! Messenger, Windows Live Messenger, Google Talk, AIM, ICQ, and Jabber.

Product Details
OS: Cross-platform (web-based application)
Genre: Instant messaging
License: Proprietary
Website: meebo.com
 

List of Instant Messengers and Chat Rooms