I’m shamelessly stealing this question from James Nicoll, who asked it about science fiction. The question is a play on the famous comment that only of order a thousand people bought the first Velvet Underground record, but every one of them went on to start a band.
So, the question is, who is the Velvet Underground of science? That is, who is the best example of somebody whose work was only read by a tiny number of people, but ended up being incredibly influential on those people and subsequent generations?
The physics example that comes to mind immediately is Sadi Carnot. Carnot wrote a little book about heat in the 1820′s that basically nobody read until after his death. It turned out to lay the foundations for essentially all of thermodynamics, though, and was tremendously influential in the future development of the physics of heat, entropy, and all the rest.
That’s just a quick reaction, though, and there may be someone better. And, of course, there are all those other branches of science that probably have their own obscure but influential historical figures. Which is why we have blog comments, after all, so leave your own nomination there.