ICQ Lite lets people log on and access contact lists from a Web site, much like Web-based e-mail accounts such as Microsoft's Hotmail and Yahoo Mail.
AOL Time Warner on Tuesday also released a version of ICQ for Mac OS X, coinciding with the release of an OS X-compatible version of its America Online service.
"The new versions of ICQ make it even easier and more convenient for users to stay in touch in real time from wherever they are and in a variety of different ways," Jim Bankoff, president of AOL Web properties, said in a statement.
This is not AOL Time Warner's first Web-based IM service. AOL launched a Web-based version of its AOL Instant Messenger, called AIM Quick Buddy, nearly two years ago. The product has since been upgraded and renamed AIM Express.
The new ICQ releases come as competing instant messaging services from Microsoft and Yahoo introduce bells and whistles such as videoconferencing in a bid to overtake AOL's lead in a fast-growing market.
By contrast, AOL has avoided loading on new IM features, concentrating instead on making its products more mobile under its "AOL Anywhere" and lesser-known "ICQ Anywhere" slogans. ICQ introduced server-based contact lists with the release of version 2001b earlier this year, in effect freeing its members from the desktop by storing address books and other data on central computers maintained by the company.
AOL Time Warner, with its AIM and ICQ messaging services, has 41.7 million home subscribers, up 21 percent from a year earlier, according to a report last month from online research firm Jupiter Media Metrix. MSN Messenger users rose 94 percent to 18.5 million; Yahoo Messenger subscribers rose 25 percent to 11.9 million.
None of the services work with each other, although efforts are under way to set common IM standards. AOL, which has long been criticized for refusing to open its network to rivals, has recently cut deals with IBM subsidiary Lotus and with Sun Microsystems to test interoperability for its service.
In the past year Microsoft has aggressively challenged AOL on several fronts, offering special deals for Internet access and integrating its instant messenger into its new operating system, Windows XP.
Microsoft plans to serve up a virtual smorgasbord of features it hopes other companies will use to add services. Windows Messenger's videoconferencing features could be used to enhance online gaming or distance learning. Software developers might also use the messaging technology's file-sharing and whiteboard features to build online collaborative applications.
Among other online services, the updated Windows Messenger will support .Net Alerts, instant messaging subscriptions that can be used to track stocks, auctions and other dynamically changing online information.