Joey makes some great points over at GlobalNerdy. He starts with this Paul Graham quote:

At this point, anyone proposing to run Windows on servers should be prepared to explain what they know about servers that Google, Yahoo, and Amazon don’t.

It’s still very true. The balance of his post is a rundown of named “Web 2.0” firms, together with their apparent server choices. It is Linux, not Windows, that runs Reddit, Digg, Del.icio.us, Flickr, Twitter, YouTube, Photobucket, Wikipedia, and a long list of others. The only top-tier site surveyed on Windows is MySpace, which explains the site’s legendary instability.