For freebies I've got a few "bigcat" instruments, both as vst and sfz that I quite like. They've usually got a very minimal interface and tweaking options compared to the pro stuff out there, but on the flip side are easy on the mind of both user and puter alike...and they sound pretty great for the most part, or at least those that I've kept. I also use the free ones that came from Steinberg, which are already listed above.
Some found here: http://bigcatlists.blogspot.is/2014/05/totally-free-instrument-by-type.html

EDIT: With this new addition I'm also rediscovering the value of soundfonts, which you can now use as replacement (Default Synth) for that awful built-in dls we have from Apple. And since the sforzando application can convert these soundfont filetypes and make use of the sounds like any sfz files, the usability of soundfonts has become even greater.
And my main go-to place for soundfonts that are (nearly) all free: http://www.soundfonts.gonet.biz/list.php


There's only one thing to do in crisis like this - Sleep on it!
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BBox 2022 Audiophile, Mac Pro Intel, OSX 10.6.8, 800x600 (TV VGA)