Domas Mituzas (who works for MySQL AB and works with Wikipedia) has published a workbook (pdf) about about the design and architecture of Wikipedia.
Expect to see lots of common infrastructure tools used in common across sites like Livejournal, Digg, etc.
Basically, everyone is using about the same core technology:
Started as Perl CGI script running on single server in 2001, site has grown into distributed platform, containing multiple technologies, all of them open. The principle of openness forced all operation to use free & open-source software only. Having commercial alterna- tives out of question, Wikipedia had the challenging task to build efficient platform of freely available components.