Annotated Bibliography of Gesneriaceae
(by L.E.Skog & J.L.Boggan, Smithsonian Institution, Washington DC)
Indispensable for search of scientific literature on Gesneriaceae
Introduction: http://persoon.si.edu/Gesneriaceae/bibliography/
Query page: http://persoon.si.edu/Gesneriaceae/bibliography/query.cfm

World Checklist of Gesneriaceae
(by J.L.Boggan & L.E.Skog, Smithsonian Institution, Washington DC)
Indispensable for search of scientific names of Gesneriaceae
Introduction: http://persoon.si.edu/Gesneriaceae/checklist/
Query page: http://persoon.si.edu/Gesneriaceae/checklist/query.cfm

Botany Greenhouses. Plant of the week
Smithsonian Institution, Department of Systematic Biology
Excellent photographs by Leslie Brothers
Query page (select family Gesneriaceae): http://persoon.si.edu/plofweek/query.cfm

The Gesneriad Society 
(formerly the American Gloxinia and Gesneriad Society, AGGS)
General information: http://www.gesneriadsociety.org/

The Gesneriad Refence Web (by Ron Myhr)
Includes over 1500 images and a number of botanically-oriented articles: http://www.gesneriads.ca/
Image index (alphabetical listing): http://www.gesneriads.ca/alphabet.htm
Search page: http://www.gesneriads.ca/search.htm
Historical illustrations: http://www.gesneriads.ca/antique_gesneriad_prints.htm

Missouri Botanical Garden: W3 Tropicos
Tropicos Image Index: http://mobot.mobot.org/W3T/Search/image/imagefr.html
Under "Gesneriaceae" a large number of species is illustrated in the form of photos of living plants, photos of herbarium specimens (types), and line drawings (esp. Chinese taxa)

Plantas brasilieiras. Brazilian plants (by Mauro Peixoto)
Photos of many Brazilian plants, including Gesneriaceae: http://brazilplants.cjb.net and http://sites.uol.com.br/mpeixoto/index.html

Taxonomie des Gesneriaceae du Bresil (by Alain Chautems)
Genera and species of Brazilian Gesneriaceae: http://www.ville-ge.ch/cjb/recherche/biomol/taxogesne.html

Gesneriads.com
Wallace Well's Gesneriad Homepage
http://www.gesneriads.com/
Basic photo gallery: http://www.gesneriads.com/gallery.html

Texas A & M University, Bioinformatic Working group
http://www.csdl.tamu.edu/FLORA/gallery.htm
Photos of Gesneriaceae: http://www.csdl.tamu.edu/FLORA/imaxxgsn.htm

Gesneriasterna
Swedish gesneriad organization, affiliated with the AGGS, but with its own international style, in both Swedish and English: http://www.gesneriasterna.se/

The African Violet Society of America
http://www.avsa.org/
Photographs (cultivars of Saintpaulia): http://www.avsa.org/Photographs.asp