A few of my favorites:
Cryogenics: Did you hear about the cryogenicist who wanted his windows cleaned? He had Bose-Einstein condensation.
Physics: Did you hear about the French postdoc who went to work at Fermilab, but never went in because the sign over the door always said it was closed?
(OK, OK, that one was more of a French joke than a physics joke! I'll make it up to you with two more!)A physicist walks into Stephen Hawking's room and says, "There's a meson your wall."
What did the proton in the bubble chamber say to the kaon? "I hope you're just passing through, stranger."
Astronomy: Astronomers do it charging, coupling, and devising.
Chemistry: Have you heard about the new oral contraceptive--hexanitrosobenzene?
(Also sometimes I've heard: What is the most chaste organic compound? Why, hexanitrosobenzene of course!)~ Miss Fleecy
PS. I just realized some of you may not be familiar with IUPAC chemical nomenclature! Hexanitrosobenzene of course has NO in all six positions!