Your favorite unusual instrument!

[sul G]

There are accordions in all manner of classical pieces, contemporary works above all. Gerhard uses one as a substitute for the baroque continuo in La Peste; Berg uses one, for obvious reasons, in the tavern scene of Wozzeck. That's to mention only two, but there are many more. But perhaps the most spectacular, in-your-face example is from an earlier work: the four accordions of Tchaikovsky's orchestral suite no 2 (it's only pro-symphony prejudice that makes these wonderful confections comparatively little known, I'm sure). Sample page below:

[Ugh!]

note the accordion in there too

[Teresa]

My favorite unusual instruments used in Classical music are:

The Banjo: Gershwin's Porgy and Bess; Thompson's The River and The Plow That Broke the Plains

Taxi Horns: Gershwin's American In Paris

The Mandolin: Mahler's Symphony No. 7

The Cimbalom: many compositions by Kodaly, Bartok and Liszt's orchestal versions of the Hungarian Rhapsodies.

The Zither: Johann Strauss Jr. Tales from the Vienna Woods

Ondes Martenot: Olivier Messiaen's Turangalîla-Symphonie

A Land Rover and Automobile in La Folia de la Spagna

I know these are very mild compared to what have been shown so far on this thread.  There are other usually instruments used in Classical music, especially orchestral music sometimes used for shock or novelty.

[DFO]

The harmonica. Listen to Gordon Jacob's divertimento for h.and SQ, James Moody's suite dans le style francais for h.and harp, or his quintet for h.and SQ. Virtuoso Tommy Reilly.
Or the fantastic arrangements of old dear Larry Adler. In particular Enescu's first romanian rhapsody.

[Cato]

Harry Partch's microtonal inventions are always interesting to my ears, especially the Cloud Chamber!

More prosaically, I have always had an ear for lower woodwinds: alto and bass flutes, bass clarinet, and English horn.

I have a recording of the unused score by Bernard Hermann for "Torn Curtain" wherein he uses a choir of 10 flutes.  He thought they produced a sound most frightening!   

[snyprrr]

the tempatation to say children's toys

the possibilities are there, but i wasn't too impressed by the kronos qrt playing terry riley's latest quartet with wu man.

also, the possibilities of paper, water, rocks in tan dun's ghost opera, though the again the execution left me cold.

but yea, how bout WATER?

[Gurn Blanston]

Bumping up for Harpo; there is a goodly part early in this thread on the Theremin if you are interested.

[Conor71]

I really quite like the sound of the Saw/Flexatone! - 2 examples of its effective use in my record collection are Khatchaturians Piano Concerto and for non-classical, Mercury Rev's album Deserters Songs.

[escher]

my favorite aside the daxophone is the bazantar
http://www.youtube.com/watch?v=crSi9IxPfYA

another one that i really like is the array mbira
http://www.youtube.com/watch?v=5fAAGheYTFA

[Mirror Image]

Don't know how unusual it is by today's standards, but I like the cimbalom, which is used in many classical compositions (Liszt, Bartok, Kodaly, Dutilleux):

http://www.youtube.com/watch?v=x2LunPxZjlY

[RJR]

My favorite is the humdinger.