This is the default package repository for GNU Emacs.
To use it, type M-x list-packages in Emacs.
(This requires Emacs version 24.1 or higher.)
Since you'll probably want to use your installed packages, it's also
recommended
that you add (package-initialize) somewhere
in your ~/.emacs file.
Or you can just browse the list of packages.
Packages are managed through the
GNU ELPA repository
(see the GNU Emacs project page
for clone instructions).
To contribute new packages refer to the
README.