Is there any hosted, browser-based software - preferably running on PHP, but I'm ready to consider other possibilities as well - that comes close to the experience of using Google Docs? I'm interested mainly in the method of organizing documents, and the minimalistic User Interface - I could live with support for a smaller number of file types.
Both Open Source and Pay software are an option.
(Sorry, I initially put this as "hosted" which I realized later is a mistake. I'm looking for a solution that I can run myself on a server.)