From Online Manual
What is the package manager?
The package manager is a tool integrated into SMF to install the custom modifications made by users. The package manager opens a compressed (zipped) file, looks for installation instructions (most commonly used is XML (Extensible Markup Language) during installation and PHP for both MySQL and after installation) and reports what can be done and what not. In short: the package manager is a great tool to work with. It is the key to enhancing SMF with all of the desired features possible!
Can I, as user, also create modifications?
Of course! You, as a user, can look at the ways of creating a modification (a.k.a package). The user can both create and host packages on your own site.
Where can I find Modifications
You can find all modifications approved by smf here. You may find other modifications in various places on the forum as well.