eGiantess - Free site, gallery is full of collage work, modifying images from other sources.
Giganta.org - Subscription-only site, designer does not originally speak English. Currently not accepting any applications for membership. Admin sounds like a dick.
Fetishlands - Acts as a portal to other GTS porn sites that sell videos.
M2 - Very slick design, but still located on Geocities so view it while it's still up (Yahoo will be taking Geocities down soon). Full of collage work, entirely oriented to the Mega variety of GTS.
Celebrities Giantess - Clunky site design, difficult to navigate, long processing time. Mega GTS collages using international models and actresses.
Midourei's Homepage - Japanese site, collection of multimedia GTS work. Lots of collected drawings from amateur giantess artists from around the Web.
WarzWars - First-generation Web design by a Japanese artist/writer. I can't read Japanese and I can't find the pictures, but I'm including this link in case you can.
Chuckbleep - Professional-grade artist with a broad gallery of diverse work. He favors shrunken men and "rubanesque" women.
Mamabliss - The artist behind Mamabliss is the jewel of sexual fetish artwork. This artist combines a playful sense of humor and shameless inhibition with his/her remarkable talent to produce comics and stories that stagger the imagination and inflame the loins. However, s/he also delves into other areas outside of large butts and gigantic women, and some of these might be too disturbing for the casual consumer.
Sankaku Channel - A more integral database of anime and hentai images. The resolution is better, the amateur artwork is better, and most work is adequately cited--you can follow it back to the source! Search for "giantess" or "huge ass" to receive hundreds of results.