Now I add another site that has impressed me by the amount of blocks it has and the way they are organized. This place is View Forum Posts and although the site has many things, such as tips and a very extensive forum, in the download section it has a good collection of blocks.
In the case of the general categories for AutoCAD they are organized in: Architecture, Mapping, Pipes, Mechanics, Transportation, Electricity. Here is a list of what you can find.
|Piping, P&ID 184