Ok this may be a foolish question, but I just want some clarification on this. If you build a website on a GPL licensed web framework, let's say a browser based game or some kind of kind of sophisticated web application are you required to redistribute all the code?
If this is so what licenses would allow you to build on top of an opensource project without requiring you to redistribute the code?
You don't specifically mention which version of the GPL license you are referring to, but this question is mentioned on their FAQ and, I think, the answer is the same for all verisons.
The short answer is that you don't have to redistribute the code in this case:
From the GPL faq, referring to a question about a company running modified GPL code on their website:
The GPL permits anyone to make a modified version and use it without ever distributing it to others. What this company is doing is a special case of that. Therefore, the company does not have to release the modified sources.