Based on this StackOverflow question, there's a possibility to develop for the native/stock Android browser. But it doesn't seem developers are latching onto creating at least some plugins for it. So for my question, I want to know if there exists some stock browser plugins already?
At least one plugin exists. Adobe flash.
Note that plugins and extensions are not the same. The default AOSP browser doesn't support extensions.
I don't know if it's what you're after as it's a big commercial plugin that's unlikely to tell you much, but the Flash Player springs to mind as the only browser plugin that I know of.