Let us tackle guitars, bass guitars, drums and electric pianos.
I try to avoid the fairly old technology of soundfonts. However Flame Studios has a nice collection of electric and acoustic guitars and basses in the soundfont format, so here we go. Many DAWs have the ability to play soundfonts, so check to see if your DAW has some kind of soundfont player. If not try either SFZ+ by Cakewalk or DSK SF2.
Flame Studios has 2 acoustic guitars, a bass acoustic, 3 electric guitars recorded in different ways and 2 electric basses all in SF2 or soundfont. This will keep you going for a while.
The Natural Concert Guitar is a nice sfz acoustic guitar. Scroll down toward the middle of the page for the download link.
D. Smolken has a fun sfz electric guitar called Cute Emily. It has all kinds of interesting settings.
For free guitar amps try Guitar Rig Player and Amplitube.
Karoryfer Samples, D. Smolken's group, has four different bass guitars. All of them are interesting and well sampled because the Dman is a profession bass player who specializes in playing in bands with supermodels. Not that I'm jealous.
Project 16 sampled his Rickenbacker 4001 and made a fine sfz instrument from the samples. I did a VST version with which the French have a love / hate relationship.
One of the best free instruments ever made is DrumMic'a by Sennheiser to show off their mics. I couldn't care less about the mics, but I love the 13,000 sample drumkit. Yeah 13 thousand samples! Bedroom Producers Blog steps you through the German language download process. A bit of a pain, but worth it. It uses the free Kontakt Player that is included.
A side note if you ever want to get Kontakt, which is the standard for sampling and has lots of free instruments available for it, download and register DrumMic'a. Then wait for the upgrade sale they have every Summer. You can get Kontakt for about $125, instead of the normal $400. Now back to your regular programming.
SM Drums are another amazing drumkit available in sfz. Their slogan is Deeply Sampled Drums for the Masses and they aren't kidding. This is a huge and excellent drumkit from a pro drummer and a dedicated team.
PTV suggested Big Mono as another high quality drum set. Get the samples, then the sfz mappings.
For a small Jazz Kit, go to Orange Tree Samples for the samples and follow their link for the sfz mapping.
On the Electric Piano front:
I like the 4Front E-Piano and R-Piano.
The MDA E-Piano has been recommended to me as very good.