There are already lots of good sites dealing with (historical) cycles, but some very interesting sources especially about geared hubs are listed here. A treasure trove for hub freaks and those aspiring so to be. The following links are arranged according to subjects and listed alphabetically within those subjects.

Generally about geared hubs
A good Wikipedia article about geared hubs providing lots of background information about contemporary and historical geared hubs.
A very nice site with various comments, exploded drawings, pictures of contemporary and historical geared hubs – and much more.
Here you will find a good review about the most important hub manufacturers with pictures and brief descriptions.
Sheldon Brown was one of the most important bicycle mechanics and the first one to present his nearly inexhaustible knowledge about bicycle technology on the internet. You will find almost everything your ‘hub-heart’ may desire.

Mainly about Fichtel & Sachs
A private site of Andree Schote about some Fichtel & Sachs 2 speed and 3 speed hubs with pictures and further useful information.
Sheldon Brown’s site especially about Fichtel & Sachs and Sram.

Mainly about Sturmey-Archer
Tony Hadland is the author of ‘The Sturmey-Archer story’ and a qualified expert on Sturmey-Archer geared hubs. You will find repair instructions of old Sturmey-Archer gears among other things.
Sheldon Brown’s site especially about Sturmey-Archer. Visiting will be absolutely worthwhile.
A great site with probably the most comprehensive collection of documents of Sturmey-Archer geared hubs in the internet. Shows all Sturmey-Archer hubs from 1902 to 2000.

Established hub manufacturers
Since 2007 a geared hub with continuously variable transmissions is available on the market! It was very heavy initially (more than 4kg), but the latest development weighs about 2.5kg only. The manufacturer provides useful information and some videos.
A real stroke of genius, a 14 speed geared hub! It is only half the weight of the Sachs Elan, it has more gears and it is successful since 1999. There exists a Wiki article as well . It’s interior is a feast for the eyes and it could even be found as an exhibition piece on industrial exhibitions.
Besides bike components Shimano also produces fishing and snowboard components, as well as forged parts. Its Nexus hubs are pretty innovative and you will find more in the Technical information section.
This legendary manufacturer of geared hubs unfortunately was forced to file for bankruptcy in the year 2000, after almost 100 years of company history and it was taken over by SunRace. The name will be continued and a great deal of hubs in different versions are being offered. There exist e.g. Fixed gear hubs and 3 speed hubs with rotary gear selector.
Unfortunately the legendary manufacturer of Torpedo hubs doesn’t exist any longer – but some successful hubs, e.g. the Super 7 are still being offered with new name and modified design by Sram.