Other people have brought up the fact that companies self-censor their Chinese-audience products, for example the classic "Tianemen Square" search on gooogle.cn vs google.com. But there is other censorship as well, they like to block sites based on IP.

To see for yourself, try out: http://www.linkwan.com/vr2/#world Click the Beijing, China location. It will do a traceroute to the website of your choice, if it is reachable from China of course! (It is a java app, warning)

For example, for www.nationalpost.com (Canadian news paper):
"www.nationalpost.com was found in 25 hops. But problems starting at hop 9 in network "CHINANET backbone network" are causing IP Packets to be dropped"

Others similarly unreachable:

www.cbc.ca (canadian broadcasting corporation)
www.freetibet.com (funnily enough just a domain squatter)
etc.

Some that work :)
www.china.com
www.xinhuanet.com (official state news agency of China)

Anyway, I found out about this when my webhost managed to get a block of their IPs banned, which prevented my hosted site (completely unrelated to the site they wanted to be banned) from being seen by my friends in mainland China since the webhost used virtual hosting to share IPs.