A great selection of free filters and Fxs

Classic Master Limiter 1.05
Very good for final mastering.
Download the plug-in HERE

Analog Industries Filterizer
A nice replacement for Steinberg's StepFilter plug-in as it can be controlled via midi. Download: Window Version or OSX Version

Bass Chorus
Split the trebble from the bass.
Download the plug-in HERE

Baxxpander
Also good for drum'n'bass or vocal tracks.
Download the plug-in HERE

The MDA Suite
30 great open source plugins for free.
Bandisto - Multi-band distortion
BeatBox - Drum replacer
Combo - Amp & speaker simulator
De-ess - High frequency dynamics processor
Degrade - Sample quality reduction
Delay - Simple stereo delay with feedback tone control
Detune - Simple up/down pitch shifting thickener
Dither - Range of dither types including noise shaping
DubDelay - Delay with feedback saturation and time/pitch modulation
Dynamics - Compressor / Limiter / Gate
Envelope - Envelope follower / VCA
Image - Stereo image adjustment and M-S matrix
Leslie - Rotary speaker simulator
Limiter - Opto-electronic style limiter
Loudness - Equal loudness contours for bass EQ and mix correction
Multiband - Multi-band compressor with M-S processing modes
Overdrive - Soft distortion
Re-Psycho! - Drum loop pitch changer
RezFilter - Resonant filter with LFO and envelope follower
Round Panner - 3D panner
Shepard - Continuously rising/falling tone generator
Splitter - Frequency / level crossover for setting up dynamic processing
Stereo Simulator - Haas delay and comb filtering
Sub-Bass Synthesizer - Several low frequency enhancement methods
Talkbox - High resolution vocoder
TestTone - Signal generator with pink and white noise, impulses and sweeps
Thru-Zero Flanger - Classic tape-flanging simulation
Tracker - Pitch tracking oscillator, or pitch tracking EQ
Vocoder - Switchable 8 or 16 band vocoder
VocInput - Pitch tracking oscillator for generating vocoder carrier input