China has the pipa, a four-stringed lute played for millenia in China and particularly popular during the Tang Dynasty. Here is pipa virtuoso, Liu Fang:
In the 7th century the pipa was taken to Japan and became various types of instruments called the biwa with either four or five strings. Here is Junko Ueda singing a traditional piece while accompanying herself on biwa:
Japan also has the shakuhachi flute as a solo instrument. It is particularly associated with Buddhist meditation.
In West Africa there is the kora, a 21-string harp. Here is Lankandia Cissoko from Senegal with traditional music for kora:
In Eastern and Southern Africa there is the very unusual instrument the mbira or thumb piano that consists of metal strips fixed to a wooden resonating box and played with the thumbs. Here is a collection of mbira music from Zimbabwe:
In Western European music there are a number of important solo instruments. Probably the oldest of these is the organ whose oldest examples go back to ancient Greece and Rome. The Roman water organ or hydraulis has been reconstructed from surviving remains and depiction in mosaics. Here is a modern reconstruction:
But we have no idea of the music that was played on it! For a long time, most music, at least that has been preserved, was for voice. One of the earliest solo instruments was the lute, developed from the Arabic oud. Here is the Syrian master of the oud, Farid El-Atrash:
Before the invention of keyboard instruments like the harpsichord, the lute was the main domestic solo instrument in Europe. Here is an early piece of Scottish lute music:
Here is a much more complex piece, imitating vocal polyphony, by the Hungarian lutenist Balint Bakfark:
The lute, with added strings in the bass, lasted to the end of the Baroque era as witnessed by this Sarabande by Sylvius Leopold Weiss:
The lute was toppled from its throne by the harpsichord, a keyboard instrument with a mechanism that plucks the strings with either leather or quill plectra. The invention of the keyboard was earth-shaking as it simplified and mechanized the production of a note. This meant that while there was some loss of control over the quality of the individual sound, many more notes could be produced far more easily than they could on other instruments. This opened the door to new heights of instrumental virtuosity. There were many schools of harpsichord playing, but probably the most important were the French clavecinistes (from clavecin, the French word for harpsichord) and the German school, especially J. S. Bach. Let's hear an example of each. I am delighted to put up a clip of my old friend from McGill, Hank Knox, playing La Poule by Rameau:
As you can see, the fact that each note is now easy to produce means that the hands can act very independently, enabling much more complex counterpoint. This was exploited by J. S. Bach. Here is the Contrapunctus I from the Art of Fugue showing how a single harpsichordist can play four independent voices.
I was planning on doing the piano, violin, cello and maybe guitar as well, but I see I have gone on very long already! So I will leave that for another day.