St. Peter's Basilica site
Lovely site at www.stpetersbasilica.org. Lots of photos of the basilica; I think it'll be an interesting photo tour even for non-Catholics because of the amazing amount of art and statuary which has accumulated in St. Peters over the ages. The site does a good job of displaying what's there in some detail. Unfortunately, the site has had some bandwidth problems recently, probably because of a lot of interest generated by the papal funeral and election.
[UPDATE: here's a similar website for the Hagia Sophia in Istanbul.]
[UPDATE: here's a similar website for the Hagia Sophia in Istanbul.]