The apostrophe is probably one of the most widely abused characters of the English language. I have seen so many people use it wrongly.
According to Wikipedia and another good article from Purdue an apostrophe should only be used in three places - to form possessives of nouns, to show the omission of letters and to indicate the plurals of certain lower case letters.
One of the most common errors I have seen is to use it for the plurals of nouns. For example, just this morning I saw a bus that had the name of the owner painted on it in large letters - "Kamineni Hospital's". Now it was not indicating that the bus belonged to a hospital called Kamineni Hospital (the possessive case). The correct text would have been "Kamineni Hospitals".
Now, I don't claim to have perfect English (I did miss my gold medal in English at school by 2 marks; Sandhya Rani pipped me to the post!) but the apostrophe is one place where I don't generally make mistakes!