Flubber is a light weight flexible framework based on MVC design pattern. It comes with jQuery plugin construct also, which means linking your application to client side is not difficult anymore and of course Its opensource.
It is designed from ground-up and designed to be
Compact: Basic skeliton is ~300kb
Scalable: Database scheme, CDN support
Easy to deploy: No installation required, just copy-paste
Multi Purpose: Design wide range of apps starting from Todo, CMS to a sophisticated web product
Multiple platforms: Multi platform support built-in
Localized: Localization is built-in
Customizable: Highly customizable because of less dependency
Extendable: Extend the behaviour of the app with easy Extensions or build your own extensions.
Note: It may not be simple to understand entire framework, its still under constant iterations. So It may look like its not developer friendly. There are many aspects of Framework which are missing, like exception handling, refined template design.