There are several interesting pages focused on gesneriads.
The most important of these is the Gesneriad Society website, which has an enormous amount of information on the culture of gesneriads, gesneriad organizations, commercial sources of plant material, etc.
Internationally speaking, make sure to check out the Gesneriasterna site -- this is a Swedish gesneriad organization, affiliated with the AGGS, but with its own international style, in both Swedish and English.
For good conversation, information and problem solving, don't neglect the Gesneriphiles mailing list. A mailing list is a kind of on-line information exchange, where folks get together to talk about their favorite infatuation. Gesneriphiles is an excellent resource, with many knowledgeable participants. The link above will take you to a page discussing the list and giving membership instructions. There are no requirements for membership other than an interest in the gesneriads.
The most widely grown of all the gesneriads is the African Violet, hybrids in the genus Saintpaulia. The African Violet Society of America maintains an excellent web site, with a great deal of information and links to a variety of resources.
A useful resource without a current website is the Gesneriad Hybridizers' Association. The GHA publishes CrossWords, which contains much useful information about hybridizing activity in the Gesneriaceae. There is a seed fund accessible to members only. Contact Dale Martens for membership information.
I'm sure there is more information out there -- let me know of any that you find. Just send me an e-mail.