Open Source & Web-Based Spreadsheets With PHP – Gelsheet
Gelsheet is a free and open source PHP application for creating web-based spreadsheets.
It is born as a module of Feng Office (OpenGoo) and comes in 3 versions; standalone, WP plugin and Feng Office plugin.
The application offers a desktop-like interface and capable of handling multiple documents with in-cell formulas, styling of content and exporting them to multiple formats.
P.S. Sadly, it lacks of a detailed documentation.