I know there is AllDataDIY, but are there any free websites? With diagrams of where things are, or diagnosing and repair guides?
I think your best bet is going to be "enthusiast" groups for your specific car type. These individual sites can often offer repair and diagnostics information, to varying levels of completeness, but I have never run across a make/model agnostic site that provides similar information. Maybe this site in a few years? :-)
Autozone has vehicle repair guides and job diagrams, but only for some models/parts.