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Browser shouldn't share its cache, except it can also deal with privacy issue.

Not a great deal of privacy problems, SSL content is excluded by default and you are just asking local peers if they have a binary matching a SHA hash.

The privacy leak might be that you are surfing the net when you should be working, however a firewall could give you that same amount of information anyway.

Offices often have caching proxies, right?

God no, nearly every caching proxy I have seen installed has been ripped out because they break the Internet in some annoying way.

What I am suggesting is a P2P self managing cache which isn't going to mangle Internet packets.

Having single points of failure is bad design in my view.

HTTP Proxies do this.