Downloading contributed modules

Do you mean something that lists available modules from inside your own drupal site ? A bit like in SMF ?
If so, I doubt it & in reality, with the plethora of modules available & continually being released, you'd be adding unnecessary server strain on your own site & I imagine on the drupal site.
As there's so many it's more logical to just browse the downloads/modules section in here & use the filter to show modules for whatever version you are using.

But in drupal 6 it does allow you to download updated modules from within your admin section.
But the installation of them is manual.