Plate tectonics. Asteroid impact mass extinction events. Immigrant aboriginal population of the Americas in the Pleistocene if not earlier. The transmissible agent or germ theory of disease, in particular as an explanation for puerperal fever (
http://www.nejm.org/doi/full/10.1056/NEJM200008243430819#t=article).
Diagnosis and treatment of PTSD as a clinical or medical issue, in particular from childhood sexual abuse.
There's a gray area in modern times from the remarkable increase in speed of discovery or investigation - in all fields, but especially those involving long journeys or great effort in data collection. Good ideas ridiculed overcome the ridicule via evidence, and evidence is often obtained much more quickly these days than in the past - a good idea will probably not be ridiculed for as long now, which asks the question of whether it is actually "ridiculed" - in the sense here - at all.
The notion that there was a layer of crude oil underlying the surface rocks, pretty deep, all over the planet, would have been one of those good ideas ridiculed had it been shown good - but it was only ridiculed per se until the money and engineering capability to check it became available, after which it was set aside. That happens faster now, usually.