Campfire has long been one of the most useful Web-based applications. Described as a "business chat" tool, Campfire lets you set up password protected chatrooms where users can share files, see live image previews, look at old transcripts and, of course, chat.
The Web app is now iPhone-optimized, carrying much of the same functionality as its desktop flavor, minus the ability to transfer some of the same files.
If you already have a Campfire site, just visit it with Safari on the iPhone and you'll automatically see the iPhone optimized version. Otherwise, sign up for Campfire here.