I wouldn't say its a secret weapon i use a lot. But a nice thing to have in your collection is Magnus Jonsson Ambience. Used sparingly its a whole new dimension. Its a free download. Its not an every day kinda thing, But its cool to have, great to use.
I have a lot of cool paid for plugins, but this for a free b is outstanding.
PS, Just got Waves Classic Compressors in the deal that's on. They are looking promising right now. The Waves Studio Classics also get a lot of use.