Cork is a fast free open-source Homebrew manager.

Cork is not just an interface for Homebrew. It has many features that are either very hard to accomplish using Homebrew alone, or straight-up not possible.

Features

Listing of installed packages. Cork has its own way of loading packages, which is around 10 times faster than the Homebrew implementation.
Knowing which packages you installed intentionally, and which packages wwere installed only as dependencies. While somewhat possible with the brew leaves command, it is often unreliable, often not listing packages that should be included.
Updating of only selected packages. Again, while possible with Homebrew alone, Cork makes it so easy you wouldn't believe it is not this simple in Homebrew itself.
Showing you exactly which packages a package is a dependency of. Super annoying in Homebrew, effortless with Cork.
And many other features! Just try Cork out and try finding them all 😉
Things that are not possible without Cork

Clearing of cached downloads.
Updating packages from the Menu Bar without having Cork open.
Seeing this much info about a package in one convenient location.
Tagging packages. This is a Cork-only feature that lets you mark any number of packages you'd like to keep track of.

License

This means that Cork open-source and you can do whatever you want with Cork's source, like modifying it, contributing to it etc., but you can't sell Cork or modified versions of it.