Sites like eMessenger are often very useful. I once memorized the URLs to AOL’s and and Yahoo’s web-based IM clients. When I started keeping my bookmarks online that made it easier. Occasionally I have found myself needing to chat without being able to start up a client. For instance, going off to a training or workshop somewhere with limited a wireless network. The HTTP port (for the web) is so ubiquitous that no one would block it. Which means services like this would work.