iPhone: Campfire is a popular web-based group chat application (we've been known to use it around Lifehacker HQ from time to time), and starting today it's available as a free native iPhone application.
The application, also called Campfire, used to be called Ember and wasn't originally developed by the folks at 37Signals (they're the people who developed the websites for the likes of Campfire and project-management application Basecamp). Ember, however, cost $12 in the App Store. 37Signals decided to purchase Ember and re-release it in the App Store for free as Campfire.
After a rather clunky login process (it loads a web page that for some reason ran really slowly for me), Campfire for iPhone does pretty much everything you'd want from it. You can chat, upload images from your Photo Library or directly from your camera, bounce between rooms, view transcripts and browse uploaded files. Support for features like push notifications aren't yet available and I ran into some bugs here and there, but if you use Campfire for work or play and you've got an iPhone, the free app is a no-brainer.
37signals Campfire [iTunes App Store]