For those who like their resources in print, check out the book Bicycle Design by Mike Burrows. I picked up the 2nd Edition by Pedal Press at a used book store, but I see Amazon carries a newer edition.
Burrows' book is a readable mix of technical material explained in layman's terms, cycling history and some irreverant commentary. HPV fans probably know Burrows already, but the book might be of interest to this group because it gives a brief but informative explanation of hub gears, specifically the differences between the Shimano and Spectro 7 speed designs.
The "Sturmey and Spectro seven-speeds are like three three-speeds in one shell," with three sets of planets that share the same direct-drive. "The Shimano, however...works in a very different way. It is like two five-speed hubs in the same shell driving each other. That is, giving potentially 5x5=25 gears less duplicates, from which the designers have selected a smooth sequence of seven." There is also a mention and a picture of the Rohloff Speedhub.
This information is probably on the internet, but such a concise comparison was new to me. I recommend the book to anyone who spends time thinking about the way bicycles work.