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Some of the plugins include an installer, but for those that don't, you'll need to place the plugin (identified by the .component suffix) into the support folder Macintosh HD\Library\Audio\Plug-Ins\Components before starting up GarageBand. Power User note: The latest versions of OS X might try to stop you from installing these plugins (it's a long story), but you'll have success if you use this trick: hold down the Control key while clicking on the installer, and select the "Open" command from the pop-up menu.
Once in GarageBand, you can access your AU plugins by creating a software instrument track, then clicking on the "Edit" tab in the right-hand Track Info column (see left image). You can choose a custom AU synthesizer at the top of the stack, and there are hardwired slots for the Compressor and Visual EQ effects, plus four remaining slots for your installed Audio Units plugins.
You can reorder them by clicking-dragging them up and down the stack, and the position will have a direct impact on your sound; sounds start at the top and work their way down. You can also use AU effects on any GarageBand track - recorded vocals and guitars, for example - as well as entire songs (the upper right Master Track tab, then the Edit subtab, where you'll find a single AU effects slot).
Synth: Podolski
Mac musicians in the know are keen to the amazing line of commercial synths and filters from U-He Software, and their free Podolski is a deep, wild source of animated, evolving pads and classic synth sounds that blows some other expensive synths out of the water - all for free!
Synth: Tyrell
Have the urge to unleash your inner Vangelis? With Tyrell, you'll be able to summon some of the darker elements of the "Blade Runner" score with this awesomely powerful softsynth, and tweaking the presets is a great way to cook up your own signature sounds. Another total winner from U-He.
Synth: Lokomotive
There was a time when you'd pay a hefty sum for a hardware synth that could pull off deep, wall-thumping bass, but in the GarageBand era, you'll score a gem like Lokomotive for the few seconds it takes to download from the web. Thick, luscious sounds galore are to be found in this processor-efficient, highly approachable beast.
Synth: Crystal
Crystal is wonderful synthesizer that's been around for years, and it's deep enough to keep you busy for eons, with intensely complex, evolving sounds perfectly suited for just about any kind of music you care to conjure. We really dig the ability to select two presets and "morph" them into a totally new sound.
Synth: PG-8X
Installing PG-8X, a sim of a classic Roland analog synthesizer, is a bit of a primitive process (you'll want to open the "Mac-AU-discontinued" directory of the included Dropbox link for the synth AU itself, and then manually download and load the preset banks when you open the plugin in Garageband), but the resulting sound is thick, classic Roland sugar.
Synth: Clap Machine
Talk about a one-trick pony: Clap Machine lets you conjure up an appreciative audience from thin air, with a variety of sampled hand clips you can trigger by pressing multiple notes on your keyboard. It's a bit silly, but if you need the sound of many hands clapping and find yourself alone, this is the quickest way to get some virtual crowd-sourced love.
Effects: SonEQ
Equalization is the primary spice of audio sweetening, and SonEQ delivers some serious vintage vibe, smooth sound, and a very straightforward interface.
Effects: SGA1566
Don't worry about SGA1566's cryptic name; this virtual preamp adds some gritty analog warmth and drive to make brittle digital tones fatten up and smooth out, a perfect way to help make individual instruments stand out in the mix, or add some sweetness to overall songs.
Effects: Nova-67P
Nova-67P is the kind of amazingly powerful tool we'd expect to see as a commercial product, but you'll be able to take parametric equalization and multiband compression to the next level with this streamlined, pro-level sonic sculptor's tool. Expect a little learning curve on this one, but it's well worth the effort.
Effects: Pitchproof
The venerable DigiTech Whammy pedal became a key tool for bands such as Rage Against the Machine and Radiohead, and the funky results of that pitch-shifting hardware are duplicated spot-on in this little red monster. Pitchproof is funky-sounding, but a great way to make guitars sound like they're a mob of angry hornets.
Effects: Multiply
Nothing quite makes guitars and voices sing like a good chorus effect, and Multiply is a totally useful, silky-quality spin on this popular type of pedal. Add it to some acoustic guitar and marvel at how it opens up and softens the overall tone of the strings.
Effects: MFreeEffectsBundle
This vast collection of effects plugins is a cornucopia of excellence with something for everyone, from equalizers, compressors, and filters to some downright bizarre stereo effects. (The excellent Tremolo is pictured here.) There's simply too much to cover here — just be thrilled and grateful that a quality collection like MFreeEffectsBundle won't put a dent in your wallet.
Effects: Voxengo Marvel GEQ
The range and quality of the many free offerings from the folks at Voxengo is rather impressive, like Voxengo Marvel GEQ, an old-fashioned 15-band graphic equalizer that lets you quickly and intuitively gain control over your treble, midrange, and bass frequencies.
Effects: Voxengo OldSkoolVerb
Another totally useful Voxengo effect is the OldSkoolVerb, which adds some nice "space" around any track — a must for percussion and voices. Actually, it helps just about anything when used in moderation.
Effects: Audio Damage RoughRider
Any audio engineer will tell you that you can't have too many compressors in your toolkit. Unlike most other compressors, this beast is very far from transparent or subtle. Audio Damage RoughRider behaves a bit more like a rowdy teenager on too much Red Bull, but lay it on top of a flat drum beat, and it'll turn it into a sonic Frankenstein.
Effects: SoundMagic Spectral
A collection of 24 plugins that delivers some of the more extreme sound-mangling abilities to be found in plugin paradise, these gems lack refined interfaces, but make up for it by delivering some seriously bizarre audio processing. From shimmering glissandos to wild time-bending delays and some downright outrageous granular filtering, the SoundMagic Spectral plugins make conjuring up instantly captivating sonic textures rather effortless.