Mike Bostocks D3 examples
Ranked in these QuestionsQuestion Ranking
Pro Updated often
Pro A lot of visual appealing examples including source code
There are hundreds of examples to look at, including the source code. This will give you a great idea of what D3.js is capable of.
Con Examples only
The D3 gallery isn't a tutorial, it only shows examples of graphs created with D3.
For someone completely new to D3, this would likely be an overwhelming place to start.