Free Instant Messaging.

Free Instant Messaging is a communicating method between two or more people through text message using internet without installing any programe. This is a cheap method to communicate if the other person is available online.

Now this is improving, as now you can send and receive single files.

Google Talk Gadget. :-

This will allow you to do Instant messaging, All of your Gmail contacts, Video previews in chat, Photo previews in chat. Once you enter to the page, press "Launch Google Talk". You will need a Google ID, if you don't have you can get there one by registering.
http://www.google.com/talk/

MSN Instant Messenger :-

MSN Messenger is a fully featured instant messaging program that you install on your own computer or one you have permission to install on. MSN Web Messenger enables you to quickly and easily use basic instant messaging features on a web browser on any computer without installing any software.
http://webmessenger.msn.com/

Yahoo! Instant Messanger. :-

Yahoo! Instant Messanger for the web Beta. This IM can work from any messanger any where. Tosign into Yahoo! Messenger you need to have a Yahoo! ID. If you don't have you can get there.
http://webmessenger.yahoo.com/

Not at home? Need to send an instant message? :-

The AIM® Express service lets you send your messages directly from a Web browser. Best of all, it's free.To run the AIM® Express service, you need Internet Explorer® 5.5 or later, Firefox® 1.5 or later, or Safari® 2.0 or later.
http://aimexpress.aim.com/

KoolIM :-

KoolIM.com is a web messenger application that allows you to stay connected with your favorite Instant Messenger almost anywhere. You need a computer with just a browser only.There is no need to get and install any software or Java plug-ins into your computer to use KoolIM.com Web Instant Messenger. KoolIM.com provides friendly, light and very easy to use web-based interface with almost all core features from the original Instant Messenger. Available are AIM, MSN, Yahoo!, Google Talk and ICQ.
http://www.koolim.com/

EBUDDY. :-

eBuddy is a free web based messenger that enables you to chat with your MSN, Yahoo and AIM buddies. No gets or registration required!
To use the web version of eBuddy, all you need is a Javascript enabled browser on your computer. You can access eBuddy from any computer or location and it even works behind a firewall. Whether you're at home, school, at the library or at work, surf over to eBuddy and login with your account to start chatting - it's as easy as that!
http://www.ebuddy.com/index3.php

RadiusIM :-

Available are AIM, MSN, Yahoo! and Google Talk. you need to register with them. This won't take no more than 30 seconds.
http://www.radiusim.com/