Installing OpenIndiana or using it for a particular task, see Using OpenIndiana
Development, building the OS, packages, and upcoming releases, have a look at Developing OpenIndiana
2011-08-14 - OpenIndiana oi_151a released!
Finally - our first build based on Illumos is out. Please see our release notes for more info, and Alasdair Lumsden our project lead has a blog post about the release.
2011-03-25 - Google Summer of Code