I disagree with Diana Lee. I grew up with a rule, one space after all punctuation except the end of a sentence. After the . ! or ? at the end of a sentence, two spaces. But this rule was created for fixed-pitch (10 characters per inch or 12 characters per inch) typewriters. Printers, who used point size with variable-width letters, never used multiple spaces. The period was adjusted to the left, and spacing works best with always using one space. So once we started using typefaces like Times New Roman and Arial (as opposed to, say, Courier), the rule for two spaces should have gone away. In fact, there is one typesetting program in which it was completely impossible to put two space together. And it created great layout results.
So, working with word-processing computers since about 1985, the correct rule has been one space, always and only. I confirmed this with master historians of type from Columbia University's School of Library Service. Chicago Manual of Style, 15th edition, concurs: Rule 6.11: "In typeset matter, one space, not two (in other words, a regular word space), follows any makr of punctuation that ends a sentence".
As today's word processors use variable-width fonts and do automatic typesetting, this rule applies to just about everything we do today.