There are a lot of real nice Garageband Software Instruments & Effects that come with a new Macintosh (for free), the Jam Pack extensions are even more awesome.
Is there a way to use them in Renoise? Like some sort of VSTi plug-in that exists?
That will propably happen when and if Renoise gains support for AUs.
I know there is a wrapper to get VSTi’s to work as AUs but not the other way around.
AUs are pretty much ‘Mac Only’ just like Direct X instruments/effects are Windows Only.
And while we’re at it, why not add support for SoundsFonts via QuickTime aswell as support for QuickTime-Midi(Really bad GM/GS soundset).
Maybe this is closer than we think? 
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Most of those cool Garageband instruments are samples. Looped, no less. Some of them are wav, some .aif (heaven knows why Apple did a bit of both, but there it is), so all you have to do is just drag them onto the provided sample slots and assign them to the appropriate notes. Some will need to be converted to wav, since the mac version (the beta anyway) intermittently refuses to open .aif files. (Has this been fixed/ does anyone else find this?)
The accoustic guitar samples are pretty good 
Sure they are good…
But there’s no simple way to layer samples within an instrument in Renoise.
All instruments that are ‘playable’ with keyboard in GarageBand are actually AUs.
There’s plenty of sampled loops(with markers to allow time-scaling).
Also there are 2 types of loops in Garageband, some ar ‘midi loops’ that use the AUs to play the sounds and the others are Sampled loops.
As far as i know there’s no AIFF support yet.
Personally i’d like to se QT support in Mac version
(ie. use QuickTime to import/export audio).