I've written a browser-based RPG before, and am now thinking about making an experimental desktop game (like Diablo II).
Do you know of any any site/blog that describes the software behind popular games?
If 2+ people think this should be community wiki, I'll do that, since this is a bit off-topic.
This are good kept secrets of the game developers because scaling such game servers is a real hard task.
So i do not think you will find much information.
I've always found the Game Architect blog an interesting read. The man behind it, Kyle Wilson, has been around for a while and ASFAIK has been involved in more games than those listed at his MobyGames page. He has at times gone into quite detailed explanations of some design issues he and his team has faced and how they solved them.
Of course if you're not mainly interested in an actual blog, Gamasutra is a good source of knowledge that many industry veterans will write for every now and then.