The Arthur M. Sackler Gallery and Freer Gallery of Art, based at the Smithsonian Institution, has started many interesting (online) exhibitions. One of them is titled, “Arts of the Islamic world”. (Once you get here, click on the link - to the right - that states “Online Exhibition
Gallery Guide - Arts of the Islamic World”).
They’ve divided it up into four sections: Calligraphy, Abstract design, Arts of the book, and Art of the object. It’s extremely interesting stuff, with quite a bit of factual information to go along with it.
The other online exhibitions are accessible here. One doesn’t have to be exclusively interested in the Islamic exhibition itself; although i found this one to be one of the most interesting, there are diverse types of exhibitions from different cultures that you can view online.