PUBLIC MARKS with tags webapp & client

AirTalkr is a multi-protocol Instant Messenger that connects to MSN, Yahoo!, GTalk, AOL and ICQ. Not only that, it also has connects to to various Web 2.0 services like Flickr and YouTube. AirTalkr runs on Adobe AIR, so you will need the runtime to use it. As of now, AirTalkr can run on Windows and Mac. Also has Web version

ILoveIM.com Web-based Instant Messenger service lets you stay connected with MSN, Yahoo, AIM, Gtalk and MySpace messengers.