Most of the virtual instruments that I use are free VSTi's and NA Kontakt libraries. Over the years, I've tried out and deleted far more instruments than I've saved. These are the instruments that I use, and also, some drums and synths that I don't use, but that are among the best free virtual instruments available (my music has only made use of acoustic sounds, so far).
I'm going to sort this by type of virtual instrument (VSTi, Kontakt library, or Other), rather than by type of instrument, because not everyone has NA Kontakt, and it's expensive. Some of my Kontakt libraries will work with the free Kontakt Player, but I don't know which are which. People who don't use Kontakt shouldn't have to wade through a list of Kontakt libraries (I know that was a problem for me before I bought Kontakt).
I also want to suggest some alternative options that I know are good, which are neither VSTs nor Kontakt libraries, such as Decent Sampler.
I'll be doing this in installments (beginning with VSTi's), and I'll try to update it every month or two with any free additional instruments that I add to my collection. If you have suggestions for instruments that you really think should be on this list, please share! (Also, please let me know about any broken links or other problems.) You can contact me via Shortwhale or via Facebook Messenger.
Virtual Studio Technology Instruments (VSTi's)
Note that BBC Orchestra Discover has special instructions (you have to fill out a survey to get it for free), whereas all the others, you can just go download (you may have to register for a mailing list or something, but you don't have to pay anything and can download all the others immediately).
Spitfire Audio Labs
Spitfire releases a new free LABS instrument every other month. Just be sure to listen to each instrument before you download it, to make sure that you want it, because you have to edit the registry on your computer if you decide to delete one later.
There is a huge list of available instruments (the ones I have are marked by a +):
-Harmonic Flights
-Cello Moods
-Michah's Choir
-Astral Forms
+Glass Piano
-Christmas
-LA Atmos
-Electric Guitars
-Siren Songs
-Ondes Musicales
-Synth Strings
-Organic Textures
-Bass Guitar
-Autoharp
-Percussion
-Pipe Organ
-Tape Orchestra
-Lap Steel
-Guitar Harmonics
+Arctic Swells
-Textural Pads
-Monochord
-Piano Pads
-Mandolin
-Granular Whalesong
-Tundra Atmos
-Wurli
-Trumpet Fields
-Rare Groove Piano
-Strings 2
-Opia
-Moon Guitar
-London Atmos
+Handbells
+Music Box
-Dulcimer
-Pedal Pads
-Modular Piano
-Peel Guitar
-Scary Strings
-Synth Pads
+Choir
-Frozen Strings
-Charango
-Electric Piano
-Amplified Cello Quartet
-Drums
-Soft Piano
-Strings
BBC Orchestra Discover is my go-to orchestra. Spitfire offers BBC Orchestra Discover for free, you just have to fill out a survey and wait for 2 weeks for the link, unless you decide to pay for it. It says "ADD TO CART $49," then just under, "ADD TO CART," it says, "OR FREE." Just click the words, "OR FREE," and it will take you to their survey, then they e-mail you the download link in 2 weeks. (If you need a free orchestra today, please continue reading this page; there are more orchestras below!)
Versilian Etherealwinds Harp CE (Creative Commons Attribution 4.0 International license; there is also a paid version for NA Kontakt with additional features)
Versilian Studios
(They have other good virtual instruments that you probably want to check out as well, including several additional free ones here that I personally haven't used. Versilian cautions that some of their older VSTi's are unstable in newer DAWs and they are therefore releasing a lot of them in SFZ Soundfont format.)
(List Under Construction)
