Epkg is a package that provides access to a local copy of the
Emacsmirror package database. It provides low-level functions for
querying the database and a
package.el-like user interface for
browsing the database.
Epkg itself is not a package manager, but the closely related Borg 1 package manager makes use of it.
The Emacsmirror is a growing collection of Emacs Lisp packages. All mirrored packages are available as Git repositories. In most cases this is done by mirroring the upstream Git repository, but if upstream uses something else, then the mirror nevertheless makes the package available as a Git repository.
One primary purpose of the Emacsmirror is to provide a comprehensive list of available Emacs packages, including packages which have gone out of fashion (but might later prove to be useful still).
Older efforts attempting to provide a comprehensive list of available packages, such as the Emacs Lisp List, over time collected an impressive list of dead links to packages which were no longer available anywhere.
With the Emacsmirror this won’t happen. If a package’s upstream disappears, then a copy remains available on the mirror. Once its upstream has disappeared a package is usually moved from the Emacsmirror to the Emacsattic, where it is no longer updated. (The Emacsattic is a Github "organization" separate from the Emacsmirror organization, but it is considered part of the Emacsmirror project.)
For more information about the Emacsmirror visit its homepage 2 and the blog post in which the current incarnation was announced 3.