Here's a few of my favourite free audio plug-ins:
Yohng W1 Limiter
George Yohng has provided an immense service in the form of a free alternative to the Waves L1 limiter. It's great for smoothing transients. For a long time I put this on every track and sometimes even the master too.
Luxonix LFX-1310
You know that tinny radio sound that sounds great on vocals or the master bus just before a big dynamic stab or for a build-up? This does that and a heap more.
Audio Damage Rough Rider
Great on drums, vocals, leads and for taming freak-out moments. Lots of character but it flattens higher frequencies, so not so great on the master.
Ohmforce Frohmage
This filter is fierce and displays a lot of the Ohmforce flavour that makes their VST effects so distinctive and desirable.