There have been a slew of them on the newsgroup
I only recall a few:
Q: What is the difference between an onion and a banjo?
A: You cry when you cut up an onion.
Q: What would happen if the banjo were left out of a bluegrass band?
A; The pieces would fail to sound the same.
Q: What is perfect banjo pitch?
A: One that doesn't hit the sides of the garbage can.
Disclaimer: The opinions expressed by these jokes do not reflect my own.