Hermit Crab Sponges: An article by Floyd Sandford about the sponges that are carried around by hermit crabs.
Introduction to Porifera: A good start on the biology of sponges, including phylogeny, life history, and ecology.
Khoyatan Sponge Page - Sponges of The Northeast Pacific: Photographs of four Classes of sponges in the Khoyatan Marine Laboratory.
Phylum Porifera (Sponges): Description of the sponges and their taxonomy, from the Animal Diversity Web.
Porifera Biology on the WWW: A central clearinghouse for sponge researchers, including a porifera listserver bulletin board and who's who in sponge research.
The Sponges of British Columbia: Descriptions of sponges by Curt Smecher.
Underwater Field Guide to Sponges of Ross Island & McMurdo Sound, Antarctica: Beautiful annotated photographs of sponges (Porifera) of Antarctica.