Audio Units

This free Garageband for Windows alternative comes packaged with 2GB worth of content, including samples, loops, and built-in virtual instrument plugins to get you started. It also offers a whopping 80 audio FX plugins to edit, manipulate and transform your MIDI and audio in any way you can imagine. Get Massive by Native Instruments and learn how to use the plugin with Ableton Live, Logic, GarageBand, and FL Studio for free. Get Massive by Native Instruments and learn how to use the plugin with Ableton Live, Logic, GarageBand, and FL Studio for free. Browse the most popular free VST and AU plugins. CompressorsView All.

Audio Units can be added to GarageBand to give you many more instruments to play with.
Here are some good sources for Audio Units

1. One of the largest makers of Audio Units and other GarageBand plug-ins is AMG.
   
2. MDA-VST: Which contrary to its name has over a dozen AU plugins includung Amp Sims, Detuner, Delay, Comperssor/Limiter, and more
   
3. MacMusic.org: a HUGE list of Freeware, Shareware and Commercial Plug-Ins.

TheGarageDoor Image Gallery

SoundFonts

There are literally thousands of SF2s available on the web. In fact if you google 'SoundFont' you'll find more hits than you could probably look through in a lifetime. SoundFonts page.
SoundFont sources:
sf2midi.com presents a massive collection: At last count over 3,860 free SoundFonts available!
Hammersound

Massive Garageband Plugin Free

Tons of SoundsFonts nicely categorized. just shy of 1,100! (Piano, Chromatic Percussion, Organ, Guitar, Bass, Solo Strings, Ensemble, Brass, Reed, Pipe, Synth Lead, Synth Pad, Synth Effects, Ethnic, Percussive, Sound Effects, Drumkits and percussive kits, Collections, Miscellaneous)

Fox-gieg
Over 220 nicely categorized SoundFonts (Bass, Brass, Guitar, Loops (not loops like GB loops, actual instruments that paly sequences), noise, Orch Hit, Organ, Perc, Piano, Strings, Synth, Voice, Wind)

Massive Plugin Garageband

The Jazz Page
Muted Trumpet, Trombone, Flute, Trumpet, Brass, Tenor Sax, Acoustic Bass, Contrabass, Harmonica, Soprano Sax, Alto Sax. Guitars: Jazzguitar, Nylon Guitar, Steel Guitar, Clean Guitar, Muted Guitar, Overdrive Guitar, Distortion Guitar, Guitar Harmonics.

Software

MidiKeys
MidiKeys is a small application for Mac OS X that presents a miniature MIDI keyboard onscreen. You can use the computer keyboard to play MIDI notes, or click on the keys with the mouse. There is also an option to select a MIDI source and see incoming notes played on the keyboard.

iDrum adds the missing ingredient to your Mac-based home recording studio - a fully programmable drum machine. It's just as easy to use as the hardware drum machines of yesteryear, but with all the flexibility of a software-based virtual instrument. iDrum integrates seamlessly with GarageBand - or you can run it as a standalone application. Create drum patterns in iDrum right alongside your existing tracks, perfectly in time, all the time. Or use iDrum as the backbeat when you're laying down new tracks. Use the hundreds of included pre-programmed patterns and rhythms or roll your own. The price is a very reasonable $49.
Download Demo - The iDrum Demo Version is identical to the full version except that it will time-out ten days after you install it. It includes Audio Unit and Standalone versions of iDrum.
Free Beats
From the same company comes the offer of 74 FREE iDrum beats for GarageBand.

So, you downloaded and installed the new macOS 10.13 High Sierra after checking that most/all of your audio software and plugins are now compatible... and then discover that Apple's own DAW, GarageBand, can't load any of your 3rd-party plugins.

If you've come across the warning stating: 'The Audio Unit 'XXX' could not be loaded for your project, and may need to be updated...' then before you throw anything at your Mac's computer screen, breathe and read on to discover a temporary solution that'll help you get around this security/sandboxing issue.

We're sure Apple will provide an update in the near future, so an upcoming macOS or GarageBand update might well be the best solution. And if you use Logic Pro X or any other DAW your plugins should load absolutely fine there. This is just a GarageBand and macOS High Sierra issue. If you're experiencing this issue like many others in the Apple Discussions forum here then you can try this trick courtesey of Two notes Engineering:

Massive Garageband Plugin Chrome

• Download & install Two Notes Sandbox Patch plugin
• Reboot your Mac
• Open GarageBand
• Add Two Notes Sandbox Patch plugin on a track, a popup will appear

• Click 'Lower Security Settings'
• This screen will appear:

• Now load your AU plugins in GarageBand on macOS 10.13 High Sierra

Our support manager at Ask.Audio, Christian Labbé, tested the above on macOS High Sierra with GarageBand and noted it works perfectly, but you will need to carry out this process every time you launch GarageBand.

But at least this is a band-aid fix over a security issue that looks like it should be very easy for Apple to solve in a coming update.

Massive Garageband Plugin Download

[Via: Two-Notes Engineering]

Become a master of GarageBand for iOS and Mac: https://ask.audio/academy?nleloc=application/garageband