PPGManager
Note: any examples shown are not affiliated with the author of the mod. They are just examples.
What does it do?
If you have any external mods/contraptions (not from the Steam Workshop), you can manage those here. Support for Workshop mods pending.
You can also package your mod/contraption in a zip file to share with others.
How does it work?
Installation
First, drag a .zip, .rar, or .7z file containing the mod.
Note: You want this: 
NOT this: 
.
Keep the mod in the root folder.
After that, it will give you a popup: (image)
and you can choose to install it. If you already installed it, you can delete or reinstall it.
When it's done installing (which should be instant), it will show this: 
Management
There are 2 buttons, 1 for mods, 1 for contraptions. The following will be looking at mods since it's only slightly different.
Click a mod on the list to find it's options.
From here, you have 3 options: Enabling/disabling (for mods), packaging it, and deleting it.
Note that the Enabled checkbox doesn't work with contraptions, since they are spawned in-game by you, vs a mod.
Toggling
Toggle the Enabled checkbox on, and it will be set to on next time you open the game. Turn it off, it's off.
Packaging
Press the Search button to locate a place to save the mod. After that, press Save to save it to that location. If you want the mod to be enabled on install, tick the "Enable mod by default" checkbox.
Deleting
Deleting will remove the mod from your hard drive. Simple as that.