Dependencies
From OpenSimulator
You may find more information on dependencies in Build Instructions though this will also contain dependencies required only for building
Windows
I don't believe any extra packages need to be installed to get the binaries running. More information on this would be most welcome
Linux/Mac OS X/FreeBSD
The easiest plaform to get running on the Linux side is Ubuntu 8.10, 32bit. This is what most of the developers running Linux use. If you are looking for the quick path, start there.
Ubuntu 8.10
You can use the built in packages for mono. However, for better performance, you may want to upgrade mono to 1.9.1 (Other simple method)
sudo apt-get install subversion nant mono-gmcs libmono-microsoft8.0-cil \ libmono-system-runtime2.0-cil libgdiplus libmono-i18n2.0-cil libmono-oracle2.0-cil ruby