meebo - the most popular third party web based instant messaging serviceeBuddy - chat with your buddies using web and mobile versions

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 for 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
Mobile IM

Agile Messenger - user friendly all-in-one application for your mobile phone

Eqo - save up to 95% on calls, 70% on text messaging, integrates all of MSN, Yahoo!,
GTalk, AIM, ICQ, and Jabber.
fring - use it as an IM client, but also for free calls over Wi-Fi, GPRS or EDGE

IM+ - All-in-One Mobile Messenger that works on any mobile device

OctroTalk - keeps you connected with your buddies, and works seamlessly over GPRS/EDGE/CDMA/Wifi/Bluetooth data connections

QuickIM - connect to public MSN networks using your cell phone with one time license fee

JiveTalk - connect simultaneously to AIM, MSN, Yahoo!, Google Talk, ICQ, and Jabber

Causerie - an easily configurable chat tool with simple-to-use user interface that work on Palm, Blackberry, Pocket PC, Symbian and J2ME enabled phones.

TipicME - J2ME-based Jabber client that is really well done and absolutely free

iSkoot - mobile application that allows you to connect by GPRS, 3G or Wifi and to speak or chat with all your contacts on Skype

MovaMessenger - a free mobile IM client, works with MSN, Yahoo, AIM, ICQ, and GTalk

Turbo MSN - a mobile MSN client for Nokia and Panasonic

Nimbuzz - call your IM buddies on their mobile or on their PC, chat with your IM buddies for free and send text messages for free.

Heysan - web based mobile messenger, compatible with most of the mobile phones including iPhone.

YehBA Mobile IM - allows you chat with your friends anywhere in the world, meet new friends across the globe through chat rooms, and have access to an unlimited number of fun services.

Palringo Vocal IM - transforms your mobile device into a walkie-talkie: communicate with a contact or a group at the touch of a button

MT Messenger - exchange messages using the most popular IM applications from MSN, Yahoo! and AOL

EpyxMobile - call all over the world from your mobile phone for SkypeOut tariffs

GoTalkMobile - Java application for J2ME enabled phones, allows you connect to Google Talk and Jabber

MSN Mobile - take MSN with you

Mundu - conference with your contacts across IM services in a single chat session