They include:
Blogger
Youtube
Facebook
Most human-rights groups such as Amnesty International, Human Rights Watch etc.
The Daily Show/Colbert Report websites
Wikipedia's entry on 'Chinese Internet Censorship'
Websites that have been blocked in the past include non-Chinese Google, Yahoo, and Hotmail.
Firefox, canny little devil, notes that when the website you're trying to access is blocked it does not say 'Error' or 'Web timeout' but 'connection interrupted'.
Of course, if Blogger is blocked, how am I writing this? I'm using what's called an online proxy, which hides your ISP and allows you to surf the internet (mostly) free of restrictions, though some websites such as Facebook are still a bit off.
