Emacs 24 by default uses the GNU package archive, which is limited to software they want on it.

However, `package.el` defines an alist of `package-archives` which you can add entries to. My hope is that then someone will end up creating a less restrictive package archive to complement the offerings in the GNU one.

Nathan Weizenbaum, of Haml fame, has done some work on a project for creating Emacs Lisp package archives and an accompanying front-end here: http://code.google.com/p/marmalade/