Broadening the discussion to something wider than punctuation (which is the subject of the excellent book "Eats, Shoots & Leaves"), I thought I would start a very pedantic thread on correct grammar and punctuation to enable the correct use of the English language
So, for starters:
Lose - as in when you lose a game
Loose - the opposite of tight
Their - as in belongs to
There - as in over there
Your - as in "I will smash your face in"
You're - as in "You're kidding aren't you?"
It's - as in "It's raining again'
Its - as in "My dog can lick its tail"
More please...