I'm using the Throttle Concurrent Builds plugin 1.7.1. Seems to work. One improvement request, though. When a build is held back in the queue because it's throttled, there's no text when hovering over it, in contrast to what's there when a build is waiting because it needs a particular executor. It would be helpful if there was some text indicating why the build is waiting (say, "Waiting due to throttling of category "Foo"), so that as an admin, I don't have to tell a user why a build's waiting.