QwertyWEB

You dont require to download these as you just have to open their websites and login to multiple accounts in just one screen ,some of them require JAVA to work ,so JAVA must be there on your OS

eBuddy - chat with your buddies using web and mobile versions

meebo - the most famous party web based instant messaging service

KoolIM - web-based IM that allows you to connect to your email inbox as well as popular IM networks

ILoveIM - another web based IM service with advertisements

IMhaha - supports Yahoo, MSN and AIM but not Google Talk

Goowy - more than just an IM client; offers e-mail, calendar, news, RSS, file storage and more

IMunitive - an alternative to meebo, but doesn’t support Jabber i.e. Google Talk

RadiusIM - web-based IM service with Google Maps integration, which means you can see where your friends are

MessengerFX - unofficial web based IM client for MSN Messenger

ICQ2Go! - web-based installation-free version of ICQ

SNIMMER - chat on MSN, Yahoo, AOL, ICQ and GTalk and make new friends by creating a profile with pictures

IMO - allows you to log in to all chat networks at the same time

Communicationtube - separate login ICQ, MSN Messenger, IRC and Google Talk contacts

Express Instan-t - simple IM which works with ICQ, MSN and Yahoo

easymessenger - free instant messenger service with built-in RSS support that works with MSN, ICQ, AIM, Yahoo! and Jabber.

mabber - use it on your phone or put it on your website

MSN2Go - Java applet that enables you connect to MSN messenger service

Yafumato Web Messenger - free and open source web-based IM client that allows you to connect to AIM, GTalk, MSN, and Yahoo

MSN Web Messenger - the web-based version of MSN Messenger, without file sharing or video calls

Yahoo! Web Messenger - access to your Yahoo! messenger, chat with your friends and get notified when new email is received.

AIM Express - official web-based instant messenger for AIM

Google Talk via Gmail - chat with your GTalk contacts from Google mail