Teambox is written in Ruby on Rails and is free to download at Github repository.
git clone git://github.com/micho/teambox.git
You can use Teambox for free in its hosted version, without worries, at Teambox.com. But you can also host your own copy to customize it!
Share messages, tasks and wiki for each of your projects. More social. Multi-language. Also the application has support for mobile phones & offers a read-only API for integrations with 3rd party softwares.