Period
More than once I've sent out plain text e-mails to a website, like http://www.michaell.org/schedule.phtml, but since the sentence ends with the URL, my instinct is to end the sentence on a period.
Of course, the problem then is that e-mail readers assume that the trailing period is a part of the URL -- so you get e-mails about sending the wrong address, or the page isn't working.
It's sort of a strange problem -- but I can't be the only one that has had it happen...

