The following artists showcase their work online:
(click here to suggest a site)

Kamel and Mokhtar el-Baba
(Beirut, Lebanon): www.ArabicCalligraphy.com

Rasheed Butt
(Islamabad, Pakistan): www.RasheedButt.com

Nihad Dukhan
(Detroit, United States): www.NDukhan.com

Haji Noor Deen Mi Guangjiang
(Beijing, China): www.HajiNoorDeen.com

Hafeez
(New York, United States): www.ArtHafez.com

M. Imad Mahhouk
(Aleppo, Syria): www.MImad.com

Hassan S. Mourad
(Vienna, Austria): www.ArabianGallery.com

Fayeq Oweis
(California, United States): www.Oweis.com

Mamoun Sakkal
(Washington, United States): www.Sakkal.com

Yousef
(Netherlands): Yucefs.ArtBismillah.org

Mohamed Zakariya
(Virginia, United States): www.Zakariya.net
This master of Islamic calligraphy also has some wonderful educational/instructional information here.

Various Artists
(Turkey & Jordan): www.irfanonline.com