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.
Homepin is manufactures multiple pinball parts including circuit boards and reproduction parts for older machines. Most parts are sold via distributor John's Arcade. Homepin also manufactures the Thunderbirds pinball machine.