For versions from Darwin 9 (the version that matches Leopard) onwards, the PureDarwin project is providing ISOs, way and code to build and use it. Currently, there is a minimal Darwin 9 iso ("PureDarwin nano", can boot into bash) and a Developer Preview with X11 ("PureDarwin Xmas", can boot into a X11 desktop).
If you are a developer, you can create your own, full PureDarwin 9 ISOs and virtual machines with our scripts and tools as described here.
For versions up to Darwin 8, Apple and the OpenDarwin project provided installation ISOs.
These are probably mainly of historical value today since the i386 version is binary incompatible to both newer versions of Darwin and Mac OS X, and hardware support is limited to older systems.
Please watch this space for changes, and have a look at the prerequisites.
This wiki is a place for users and developers alike to document and discuss Darwin.