A robust package management system is quite important when developing in Unity. People have been asking for one in the Unity forum. The following survey will list some popular options.

Projeny The purpose of Projeny is to allow your Unity3D project to easily scale in size without heavily impacting development time. Only available in Windows

• Share any Unity assets (code, scenes, prefabs, etc.) across multiple different Unity projects without copy and pasting
• Instantly switch between platforms
• Easily upgrade or downgrade installed asset store packages
• Optimize compile time of your project by getting Unity to only recompile the code that changes most often
• Split up your project into discrete packages, so that you can manage the dependencies between each, instead of having one giant Unity project of inter-related files
• Declare dependencies between packages, so that you always get the packages that you need without needing to hunt down missing libraries or broken links
• Generate a more intelligent Visual Studio solution than the Unity default, using package dependencies to create csproj dependencies

NPM can be used to manage packages and their dependencies

• It's very easy to use and install.
• Can be used with private repo
• Cannot distribute .unitypackage file
• However, without using private registry, it cannot force resolve to the latest versions of all dependencies. This issue can be resolved using yarn install --flat

Some people suggests using NuGet

• Seems can download packages correctly
• Have to install a central server to make it work

Paket.Unity3D

• Same problem as nuget
• Seems to work better in Windows

Unity Kaizen

• Deliver packages via a central repository and zip
• Require more work from package maintainer

UPM

• UPM is a package manager designed to work with the Unity game engine. It allows rapid install of unity extension and assets using a command line interface. Packages support dependencies, which will be auto-downloaded when installing a package.
• Seems to be unmaintained
• No MacOS binary

unity-packman

• A command-line client to manage Unity package
• Does not support version and nested versions
• Can only export 1 directory, which is really limited