The Tzychen blog has an interesting post on unusual web servers, including many novelty micro-servers that make blade servers look like heavyweights. I shudder to think how many of these you could pack into a rack or cabinet.
If you’re concerned about powering miniature servers, you can use potato power. Some years back Frederic White decided to see whether an online joke page about a potato-powered server could be translated into real-world working technology. White created the World’s Lowest Power Web Server, running on a single AAA battery and a bank of potatoes. Yes, there’s documentation.