The following are some interesting collections of Islamic art on the internet:

Art Islamic
A diverse blend of traditional calligraphy and more modern approaches

CalligraphyIslamic
The web’s most impressive effort to preserve and promote Islamic calligraphy

Islamicity (I & II)
Two pages of links to different types of Arabic calligraphy

Rahila’s Islamic Graphics Archive
The most extensive collection of Islamically-related digital images on the internet

Abuna Shaykh al-Bukhari
Thumbnailed images of extraordinary artwork by a variety of artists. The webmaster has been notified of the broken links.

The Arts of Islam
The calligraphy gallery of a wonderfully done online museum of Islamic art

United States Library of Congress
 Pictures of over 350 calligraphy sheets ranging from the 9th to 19th century. Images are often included by beautiful translations

Ahlul Bayt Digital Islamic Library Project
A thumbnailed collection of colorful calligraphy artwork.

(click here to suggest a site)