There are a number of online resources available that keep track of a number of Blender Addons, though I am not aware of any one centralized site that covers all of them.
Your first go-to resource is going to be the Blender Wiki's Scripts page, which keeps up-to-date information on all of the addons that come packaged with the Blender installation, as well as several others.
Another valuable resource is the unofficial Blender Addon List, a listing of many Blender addons maintained by Daniel Banasik.
Yet another resource is Graphicall.org. I am aware of a number of different user compilations of various scripts, but perhaps the largest of these is on this page.
The short answer to your question is that these resources are scattered everywhere, and while there are some that cover a large number of Blender addons, there isn't yet one location that keeps track of all of them.
Furthermore, the process of pulling these resources into one centralized location would be a logistical nightmare, with whole hosts of differences between how different scripts handle updates, as well as certain scripts being made obsolete by updates to new versions of Blender. I won't say it can't be done, but it would certainly be difficult to keep track of those things any better than the resources already out there.
Edit: zeffii also pointed out that you can get a zip with the most recent version of Blender and all user-contributed addons at builder.blender.org
Update: Added the Blender Addon List and removed a link to an inactive resource.
