Notifu

radiusIM is Social IM. It’s the only website that shows you where your friends are hanging out and lets you surf for other people based on geographic location. Your friends will show up on the friends list as icons on a map.  It supports all the major networks: MSN, AIM/ICQ, Yahoo, and GTalk/Jabber. And you can use radiusIM from anywhere because there are no downloads.

SNIMMER is a chat site and a social network. You can chat in a group with all of your friends at once or chat individually via Web IM or regular IM. Through Snimmer you can chat on MSN, Yahoo, AOL, ICQ and Google. You can also create your profile, add your photos, chat with your friends, make new friends, connect and have fun!

Easy Messenger - free instant messenger service that runs right from your web browser. Combine your MSN, ICQ, AIM, Yahoo! and Jabber contacts into one solution. Now you can use your instant messenger at school, work or just at home without having to install software.
With the build-in RSS reader you are always up-to-date with your favorite RSS feeds.

The Google Talk gadget is a free web-based application with some fun features for communicating in real time.
Instant messaging — Chat with all of your Google Talk and Gmail contacts in real time.
No download required — Start chatting immediately from any computer.
Group chat — Invite multiple people to chat with you in a group. Learn more
Media previews — Cut and paste video and slideshow URLs from YouTube, Google Video, Picasa Web Albums and Flickr into your chats and view them in your chat window. Learn more
Add to iGoogle — You can add the Google Talk gadget to your iGoogle homepage to put chat alongside other useful and entertaining gadgets.
Add to your own webpage — Just cut and paste this code into your own webpage or blog to embed the Google Talk gadget.

AIM Express is the web-based version of the popular AOL instant messenger.  Send IMs to your friends without installing any software. The AIM Express® service is convenient whenever you’re away from your home computer: at school, the office, or a friend’s house.  Talk to your friends from any computer with a Web browser and access to the Internet. You can chat at work. You can chat at school. Productivity is overrated. Bring all of your buddies into one conversation.  Keep tabs on what all your friends are doing with just one click. You never know what they might be saying…

Mabber developed a mobile application that lets you chat and send messages wherever you are. Required is a recent mobile phone equipped with Java and some sort of data-plan.
No installation required, easy to use at school and at work – mabber runs in your browser.
Jabber-compliant, just use a messenger like Spark, Pandion or Psi if you prefer standalone programs.
Chat with your friends at ICQ, MSN, AIM, Google Talk, GMX messenger and others.
Use it on your mobile phone
Put it on your website
Stay up-to-date with mabber Alerts

Messenger FX is a web-based service that lets you access the MSN instant messaging network without having to install anything on your computer. The access is protected through encryption and the interface is available in multiple languages. There is no possibility to talk to contacts that are on other IM networks. Free to use.

Free video chat. No cost. No prepaid minutes. Really free.
No downloads. Talk through your browser. No chat client needed.
Any website. Embeddable on all websites and social networks.
Privacy control. You control who can see you and who can’t.

 Tokbox

meebo.com

The web-based Yahoo Messenger is probably the most powerful rival for the famous Meebo or other similar services like Ebuddy.