Try the Great Chinese FireWall at Home Now!
I found this on the China Law Blog and Sinosplice. There's a new Firefox add-on that allows users to experience what it's like to surf the web in China. I haven't tried it myself, having lived in China a year and experienced the frustration first hand.
Apparently the web situation is better now (apart from spying on Skype users, etc.) and it's possible to access parts of Wikipedia and more blogs from within China than it was a few years ago.
From the add-on description:

The Firefox add-on China Channel offers internet users outside of China the ability to surf the web as if they were inside mainland China. Take an unforgetable virtual trip to China and experience the technical expertise of the Chinese Ministry of Information Industry (supported by western companies). It's open source, free and easy.