The world's largest collection of user contributed patches for synthezisers and effects can be found here: PatchArena - Downloads.

How it works:

You must be registered to access PatchArena - Downloads. You can rate, vote, and place comments on all files. Further discussions can be made on the PatchArena - Forums.

Register:

To get starter REGISTER HERE. Once you are registered you can start uploading your patches, skins, and sounds and start sharing them with your friends.