Pinside is the largest pinball community website in the world. There is a huge amount of expertise and knowledge on Pinside and it is a good way to stay up-to-date with pinball developments around the world.
Clay Harrel's repair guides were, up until several years ago, freely available for anyone to view. Now only a subset of the original repair guides are accessible, but still contain a wealth of information, particularly on EM and Gottlieb machine repair.
Pinwiki is the most accessible and comprehensive compendium of pinball repair information currently on the internet. If you're having a problem with your pinball machine, its fix is likely already documented here.