Kvarn is a next-generation web server designed for performance and flexibility.
Check out kvarn.org for more info.
All of my websites currently use Kvarn and I intend it to handle all use-cases. Already, it can handle most. Extensions can be used to extend the capabilities. Mail me for extension requests.
It features a pluggable extension system ontop of which many great features are built on, both internally and through Kvarn's extension library. See the Kvarn features for a list of extensions built using the extension library.