save-packages

This is a little helper to synchronize installed ELPA packages
across hosts.  Call `save-packages' on one host, copy the output
file to another host, and call `install-saved-packages' there.

You could call `save-packages' from an idle timer or
`kill-emacs-hook'.  And if you are slightly daring, you could try
calling `install-saved-packages' from your initialization file.

If you need something more powerful than this, you might want to
look into el-get.