This is an amazing resource with an extensive library filled with photos, models, sketches covering all aspects of architecture and architectural history, if you're interested here's the link.
This website isn't specifically about architecure, more art in general the reason why I put it in this list is because there is a page that gives a consise and interesting overview of architectural history. Take A Look!
Another site similar to RIBA and it's generally a pretty interesting reference site.