At the end of a sentence, actually, one ought to, if one cares about being correct, put four periods. One period to indicate the end of a sentence, and three more periods to indicate the trailing off or not finishing a thought. Two examples follow.

"What do you want to do today?"
"Oh, I don't know...."

"What do you want to do today?"
"Oh, I don't know ... maybe I'll kick your ass."

In the first example, we the readers are to assume that the person answering did not give an answer to the question at hand. He or she said he or she did not know the answer, and stopped. In the second example, though, the respondant did give an answer, albeit belatedly. The ellipsis in his or her response lets us know that there was a pause before the answer was given.

The point is, if you want to put an ellipsis at the end of a sentence, you ought to put four periods.