A couple of favourites from college and friends working in Science (apologies in advance!)

2 Hydrogen atoms walk into a bar. 1 stops and says:
“Oh no, I’ve lost my electron!”
“Are you sure?”
“Yes, I’m positive.”

A bacterium walks into a bar, and heads towards the barman.
Barman: “Oi, no bacteria allowed in here.”
Bacterium: “It’s OK, I’m Staph!”

A guy walks up to a barman in London.
“Can I have a pint of adenosine triphosphate?”
Barman: “Sure, that’ll be 80p”

Bear walks into a bar and asks for a beer
Barman says “we don’t serve bears, get out”
Bear gets angry, says gimme a beer!
Barman still refuses
Bear says “if you don’t serve me I’m gonna eat that woman in the corner”
Barman still refuses
So the bear eats the woman and returns to the bar
“now gimme a beer!”
Barman says “we don’t serve drug addicts, get out”
Bear goes what are you talking about
That was the bar b1tch you ate

(hattip to xkcd (pic) and @tomlau)