Just to prove the openness of Wikipedia’s founders. They posted their server architecture on their site, as well as a description of their hardware.
Interesting to note the evolution, from a single MySQL DB, 6 apache web servers and 2 squid cache’s to it’s current structure of multiple servers on multiple sites. (Layout history here)
Interesting to note also that their servers are named after famous encyclopedists…. of course now they also use plant names.
There is a disclaimer however that one shouldn’t take this to be always up to date as well as entirely accurate.