There are many older instruments which only exist in museums today such as the shawm, viol, cornophone and crumhorn. Others, like the Hurdy Gurdy, have been around since the 11th century and still exist but are rarely heard. The following web sites describe various antique instruments and have sound samples for some of them: Renaissance Instruments, Guide to Medieval and Renaissance Instruments, Edinburgh University Collection of Musical Instruments, Zzounds renaissance instruments. Here is a performance of a Hurdy Gurdy.
Musicians continue to make new instruments and modify older ones. Here are a few examples:
As an example of some of the complexities of percussion instruments, here is an article on Hang Acoustics. The Hang is a relatively new instrument similar to a steel drum but played by striking it with the hand.
Here is a web site that discusses how to create and play a pitch-bending accordion.
Wikipedia on the Jew’s harp.
The Theremin is an interesting instrument that uses electrical capacitance to change an electronic circuit to make sound. The performer does not actually touch the instrument as seen in the following videos. There are also video versions that capture hand movements via the computer camera and generate sound corresponding to hand motion (search for Theremin Camera).
Chinese singing fountain bowl.
Wine glass harmonica.
Tibetan singing bowls (without and with water).
The Tromba-marina, an interesting instrument popular in the 18th century.
The Stroh violin.
Here are a series of Ted talks about unusual instruments and performances.
If you ever thought of making a unique instrument that never existed before you might want to take a look at the Experimental Instruments Web site for ideas.