I would like to add some plugins to Eclipse, but I don't know the site. I clicked "Install New Software" then "Add" but I don't know which site to point to. Any pointers?
Actually, you probably have the main sites already, the official ones from Eclipse team, with mainly the Eclipse program (for updates), and related plugins.
There is no such thing as a "main repository" for Eclipse plugins that you would add once, and find all plugins from it (that I know of, at least). This "Add site" is to use for each individual plugin which allows you to download through their update site.
Here you can find many plugins, as told by heavyd in comment: http://marketplace.eclipse.org/. You can see that for most of them, you have the "update link", which is the one you would use in this "Add site".