Putting everything in the browser window — word processing, spreadsheets, photo editing, heaven knows what else — can raise more problems than it solves. Locally executed software, for the most part, ain’t broke.
But there are some tasks that aren’t terribly processor-intensive and don’t require storing important data, that take well to browser living. Instant messaging, for instance, is very pleasant using Meebo, a web application built on the Pidgin project’s libpurple library. It has taken off, I suspect, not just because it’s easy but also because it’s more workplace-friendly than installing one or several dedicated IM clients.
Now, for IRC, there’s Mibbit. It’s convenient, with a clean, usable interface, and it offers a nice helping of features. There’s even a mini-client that can be embedded into your site.