"At their best these images express a refined and even sublime aesthetic sensibility, but they always remain perfectly accessible."Four thousand free images of themes, motifs and designs from Islamic Art. All downloadable, with accompanying analysis, historical significance, bibliography.
Go eat your heart out.
Update: Via Feanor, an excellent link to an analysis of the types of symmetry found in Islamic art.
Second update: Rahul sends in another link that looks at the intersection of Islamic Art and Geometry. Great find - thanks Rahul!