SecondLife's Architecture is Open Source.

From Second Life and Open Source:

..Each "sim" or simulator of a portion of the virtual world in Second Life is created on a server running Debian GNU/Linux, Apache, Squid and MySQL; currently there are several thousand of these PC boxes. To allow for fast response times, the virtual world is sent not as pixels or even as a mesh, but as a series of 3D primitives - "prims". The Second Life client creates the world by converting the stream of information about prims and their position into a visual representation...

I thought so. I'd thought, but never asked about the architecture in this regard... I guess Glyn Moody asked (no source that I saw). 4 sims per server - Apache. Mass administrative changes across a network - Linux. MySQL - Asset servers, etc. Sims are probably load balanced by land.

The rest of the article is well worth reading as well, as it provides a thankfully sane version of the entire [t:Copybot] incident which incorporates quite a few different perspectives. But given that SecondLife is built on Open Source, well... I imagine that may have a few people gnashing their teeth.

I'm smiling. :-)