At a clients site where I have IT access, they pretty much block anything relating to forums, gaming, pr0n, ebay, facebook, hotmail, Gmail etc etc. But we can get Youtube. Go figure. Often sites are blocked on their address name. For instance I can use the OCUK forum, but other forums that use the word 'forum' in the address bar are blocked. Likewise any address using the word 'game' is blocked.
Pretty restrictive really, but then the IT system here is huge/multinational so security is pretty tight (so I am led to believe).
Our HQ IT system is unrestricted as far as I know.
To be fair though, the computers are there for work purposes, not recreation so I cant complain.