I've made an interactive list that helps you find which viewers (etc.) are bundled with Opus and which add-ons are available to make Opus do more.
The list includes several plugins/features not mentioned here at the forum. (It also mentions some of the built-in functionality that doesn't depend on plugins.) Screenshots are included, where useful, to give you an idea of how things are before you install anything.
August 2018: Note that the list is quite out of date at the moment. I may update it in the future, time permitting, but it's still useful as it is; just remember it is not complete, the same as the forum's list of plugins.
You can filter the list to show just what interests you. For example, you can make it show only plugins which support 64-bit & USB mode, and don't come bundled with Opus, so that you don't waste time looking at things you already have or cannot use.
Maybe you'll discover something new. Hope it's useful!