el-get - GitHub
Short Story: #el-get allows you to install and manage elisp code for #Emacs. It supports lots of differents types of sources and is able to install them, update them and remove them, but more importantly it will init them for you.
That means it will require the features you need, load the necessary files, set the Info paths so that C-h i shows the new documentation you now depend on, and finally call your own :post-init function for you to setup the extension. Or call it a package.
Cet outil pour gérer les scripts et extensions pour Emacs est simplement génial. Je viens de rédiger une petite introduction en français :