Pico is a stupidly simple, blazing fast, flat file CMS. See http://pico.dev7studios.com for more info. It does, however, have a problem: it scans the entire content directory hierarchy on every single page request. This is OK, when there are only a small number of pages but it effectively kills performance when there are hundreds of content pages.
To overcome this problem, I created Hippycache. It's a rather stupid way of caching content: content is stored in a single variable embedded directly into the index.php file. Yes, it works. No, it's not pretty.