Django | The Web framework for perfectionists with deadlines
Django is a high-level Python Web framework that encourages rapid development and clean, pragmatic design.

symfony | Web PHP Framework
symfony - open-source PHP web framework

Thematic, A WordPress Theme Framework
Very flexible customizable WP theme. http://themeshaper.com/thematic/

BrowserCMS
BrowserCMS is a general purpose, open source Web Content Management System (CMS), written in Ruby on Rails.

Lift – The Simply Functional Web Framework – Home
Lift is an expressive and elegant framework for writing web applications. Lift stresses the importance of security, maintainability, scalability and performance, while allowing for high levels of developer productivity. Lift open source software licensed...

Grok - A Smashing Web Framework — Grok
Grok is a web application framework for Python developers. It is aimed at both beginners and very experienced web developers. Grok has an emphasis on agile development. Grok is easy and powerful.

Rango: A New Modular Ruby Webapp Framework
Of which NYTimes.com development manager Aron Pilhofer said "Love Django but not Python? (Me me me!) Maybe Rango is for you."

Ajax.org - The real-time collaborative application platform
Ajax.org Platform is a pure javascript application framework for creating real-time collaborative applications that run in the browser.

Use Appcelerator Titanium to build mobile apps for iPhone & Android and desktop apps for Windows, Mac OS X & Linux from Web technologies
An interesting platform for deploying web apps as true apps on iPhone, Android, Desktop, etc.