Hands down my favourite freebies are a pair of Klanghelm compressors.

http://klanghelm.com/contents/main.php

I love them so much I quickly coughed up the measly 24 euros to buy the full version of the MJUC and that is now my favourite PAID plugin. Finally I understand why the pros refer to compressors as musical instruments in their own right.

I'm also loving Izotope's neutrino referred to in a post below. It doesn't do much on already well balanced tracks (eg BIAB realtracks) but it's a sure-fire quick fix for the rest.

Matcham