What instrument has the lowest pitch?

Of the standard western musical instruments, the piano seems to have the lowest pitch, at 27.500 hz, though a synthesizer can go an octave below that.

Possibly you meant an instrument that can play only one note at a time. The double bass violin and tuba can go as low as 41.203.

Too many references to cite. Here's one:

http://solomonsmusic.net/insrange.htm

The contrabassoon can, with a bell extension, produce a low A, equivalent to the lowest note on the piano. The Bösendorfer Imperial Grand piano is equipped with 9 extra keys in its bass register, extending its range to C below the normal pianos. Certain contrabass clarinets and instruments invented by Adolphe Sax (not saxophones) can reach notes that are lower still, though they are impractical since the average human ear can only tell between distinct pitches above about 20 Hertz.

An church or concert hall organ with 32 foot pipes can get down to 16 Hz. At that level, you feel the vibrations rather than hear them. Notes like that are used to give spooky effects, as they can induce feelings of unease. If prolonged, they can give rise to fear without the person knowing what they're afraid of!

Just thought you'd like to see this, although it's not an instrument some black hole is emitting a B flat 57 octaves below middle C ....that's pretty low

http://www.nasa.gov/centers/goddard/universe/black_hole_sound.html

All brass instruments except perhaps the trumpet has a pedal tone, the fundamental in the overtone sequence which is the lowest open note on the instrument. Most symphony orchestras us a tuba piched in C, whose fundamental is the lowest C on the piano. Adding the second valve lowers it to a B. The first valve to a Bb, the first and second to the bottom A on the piano. This continues until all 5 valves (most orchestral tubas have 5 valves) are depressed resulting in the first C below the bottom of the piano. The low notes are difficult to play. As a tuba player, I have been down to the F below the bottom of the piano before hyperventilating. Most pro should be able to bottom their instrument out. In fact in his album Roger Bobo plays the tuba, he actually plays the subpedal C. SO the answer is the tuba since a pipe organ and extended Bosendorfer are not standard orchestral instruments.