Currently most users of Balloon are using it with GNU/Linux. A couple are using it 'bare metal' with just bootldr providing 'setup and OS services'. We would welcome ports to other OSes (such as eCos, vXworks, WinCE), but right now you get Linux or Linux.
The sources are all stored in a central subversion repository.
If you have a developer account then read write access is available over ssh at svn+ssh://balloonboard.org/home/balloon/svn/.
Binaries for the various boards and releases are available by http or using rsync from balloonboard.org::balloon
The current Balloon3 debian-based stable release is available
There is also a development release available, which is close to stable release (Dec 2008).
Most developers are using the cross toolchains provided by Emdebian. Some tarballs suitable for developers on non-debian machines will appear on this page shortly.