Resound is a DJ/Producer from Finland who was kind enough to post on his site a list of 25 excellent plugins that are totally free. (Some may require registration on the developer's website).
The above pic is of Audio Damage's Rough Rider compressor which despite being free is actually a good sounding compressor.(Btw that's an actual setting I'm using on the drum buss for a current project).
"Rough Rider can be tweaked for really mild compression or serious hardcore pumping action equally. Rough Rider is not exactly a transparent compressor – it adds a bit of vintage style coloring to the sound. Great plugin."
I already have some that I use on a regular basis such as Camel Audio's CamelCrusher.
CamelCrusher "is a free 'colouring' multi-effect plugin. It offers two characteristically different distortion sounds which can be blended together to create a wide variety of tones and textures. Great for guitars, drums and plenty more!"
I use it often on drums, bass and to "fatten" synth/pad sounds.
The list also includes Virtual Instruments such as TAL Audio's NoizeM4k3r in addition to audio processing plugins:
"The NoiseMaker is a very versataile, phat sounding and traditional analogue-modeling synth. It’s fun and easy to tweak but also comes with whopping 256 presets. This synth can do some serious bass – great for layering under reese or stab sounds for example. Sound quality wise it easily beats many commercial synths out there. I especially like how the pulse wave oscillator sounds."
See the entire list at Resoundsound.com. Download links are included with each plugin on the list.