GreatFire (自由微博)

A group that produces tools and websites against censorship in China, central website: https://greatfire.org/

Backed by Radio Free Asia (RFA, 自由亚洲电台, USA government media) and therefore USA Government.

The GreatFire Analyzer is their most important tool, it checks if websites are blocked or not in China:

Alternatives from other providers:

It also keeps a calendar of previous checks so you can see when a website started getting blocked.

One of the highlighted collections is the Alexa Top 1000 domain block analysis: https://en.greatfire.org/search/alexa-top-1000-domains

Quite a few amazingly named porn ones, how could you live without them? Pornography ban (色情禁令)

And some interesting random stuff that is hard to understand why its blocked, some of them appear to be pure protectionism:

Other tools and projects:

Who is behind them: