IncPlugins

IncPlugins is a plugins (surprise!)-based web site engine. Free and commercial versions.

From the website:

IncPlugins is a scalable plugins-based web site engine for PHP with dynamic pages, CSS and JavaScript. Free and commercial editions are available.
IncPlugins Main Features

* Plugins-based extensible architecture
Each plugin includes PHP, CSS and JavaScript code in the same source code file.
* Plain text page sources
Plain text wiki-like transform language. Correct typographic, highlighting;
* Separation code from text
Page design, layout and page elements are located in plugins.
* Multilingual web sites
Different pages or the same page, localized text resources.
* Multidomain web sites
Mix different web sites on the same hosting account.
* Performance
Web browser cache control, server-side page caching, gzip compression.
* Core web site elements
XHTML frame, relative links, GZIP compression, cache control, page templates.

Meh. Without a big volunteer support community to back it, you're kind of painting yourself into a corner with this kind of solution, in our opinion.

UPDATE

Service is no longer available.