GameMaking Resources is one of the many sites offering free resources for Game Maker projects.
Handily the service is more dynamic than most sites which tend to just provide page after page of alphabetical resources listings. You can comment on and rate resources and a handy indicator specifies whether or not you need to give credit to the resource’s creator during your game. You can choose to view resources that have been created for specific versions of Game Maker, obviously for sprites and sounds this will not be an issue, but for scripts and examples this could be a useful feature.
There is a bot that was indexing the GMC looking for resources, however since it doesn’t appear to be adding spidered files to GM-Resources I don’t really understand its purpose. With around 300 members the site does look promising providing it keeps adding new resources, although obviously there is a balance to be struck between the quality and quantity of the resources on offer.