Snow2[1] is a simple packaging format for Scheme libraries.
You can browse the available libraries online in the packages list, and download and install them manually, or you can use existing tools to automate the install process. One such tool is Seth Alves snow2-client, which supports multiple implementations. Another tool is the snow-chibi command, distributed with chibi-scheme as of version 0.7.3, which also supports multiple implementations. This document describes snow-chibi.
From either client you can access multiple repositories. Library authors can choose to host libraries on their own servers, but it's better to publish them to a central repository so other programmers can find them more easily. A few public repositories are available, but this document focuses on the default snow-fort.org, which allows third party publishing.
Read on to find out how to install and write and publish libraries, or read the specification to create your own tools.
The full snow-chibi command reference is included in the chibi-scheme manual.
[1]The original Snow was designed around R4RS Scheme, whereas the new version is based on R7RS Scheme, which has its own library system. We sometimes just say "Snow" when referring to the new version.