O Ubuntu usa o gdm3 hoje em dia e lá é chamado de extensões do gnome . Você pode encontrá-los no extensions.gnome.org . A partir do primeiro link:
Extensions
The GNOME Shell extension design is designed to give a high degree of power to the parts of the GNOME interface managed by the shell, such as window management and application launching. It simply loads arbitrary JavaScript and CSS. This gives developers a way to make many kinds of changes and share those changes with others, without having to patch the original source code and recompile it, and somehow distribute the patched code.
The extension system is a replacement for the "applets" or "widgets" system of GNOME 2. Extensions can add their own items to the top panel, but they can also do much more: adding a custom search provider, for instance. For more information, see this post.