Developers who want to work with Campfire should check out Tinder, the unofficial Campfire API. We use it to send Subversion check-in notices and Capistrano deploy messages into our development chat room.
This turns out to be extremely useful. Not only are all the updates going to the same place; you also get to see how the checkins and deploys relate to the conversation. Awesome.
Do you use a 37signals product in an interesting or noteworthy way? Let us know.