Q:

What is the right level of site blocking for you as a developer? 

Almost all developers who work for a large corporation find themselves on the wrong side of site blocking software.  It can be both frustrating ("Just let me read that damn blog!")  and helpful ("Woah!  Dodged a bullet with that site"). 
In your opinion, what is the right level of blocking to apply to developers and why? 

A:

No site blocking. 
If my projects are delivered on time and my productivity is not suffering, I don't see any reason to block anything (except - if you really must block something - well known spyware/malware sites). 
I don't really have anything else to add except that. 

We are professionals, not children. 

A:

No blocking at all. 
Good developers
Those will get the job done, they know how to find information quickly and they also know how to use that information.  Of course if you don't give them enough to do, they'll waste their time over at the Stackoverflow Chatrooms :P
Bad developers
Well you can't do anything about them.  If you block 90% of the interwebz just because they can't focus on their work, you'll only hinder the good developers... they will leave and you're left with the bad ones! ( well in a perfect world...)

A:

Zero. 
If you have a developer who doesn't produce, blocking websites won't help that. 
The quantity and quality of code produced by a developer is a not linearly scalable with time spent staring into an IDE. 
Being productive in creative and challenging work requires "contiguous time" and "flexible time."  Contigious time is a block of time where a developer knows he won't be interrupted by anyone, flexible time is time where a developer knows he is allowed to take breaks on his own schedule and rest his mind when he needs it. 
Over management is a problem for contigious time, site blocking is a problem for flexible time.  
As Jason Fried points out in this tedTalk, ten years ago no one had a problem with employees taking breaks to smoke.  Facebook and the like are just modern day version of the smoke-break - they're necessary, useful and should be embraced by management.