Download
From OpenSimulator
Stable Releases
These are the last version that were considered "stable" and useable in a test enviornment.
Version 0.4.0
(Source) http://dist.opensimulator.org/opensim-0.4.zip
(Source) http://dist.opensimulator.org/opensim-0.4.tar.gz
Nightly Builds
These are compiled packages created daily from the svn repository. The EXE's can be run natively under Windows, but requires Mono to run under Mac OS X and Linux.
They can be downloaded from:
(Binaries) http://ruth.petitbe.be/build/
SVN Checkout
To get the very latest (and usually unstable) source, run:
svn co http://opensimulator.org/svn/opensim/trunk opensim
SVN can be downloaded for Windows from here, or for Mac OS X here.