From kottke.org reader Ed comes this timely* bit of humor. The U.S. Naval Observatory Master Clock, accurate to within 100 picoseconds, offers a RealAudio stream of the Master Clock Voice Announcer. Due to the buffering time of the audio, the "audio time will be delayed (late) by at least four seconds from when it left the USNO Master Clock time." So, I guess you'll always be able to check the exact time from approximately four seconds ago.
*Get it? Timely? "Time"ly? Ha ha ha! Fortunately for him, Ed is not responsible for that horrible pun. That's 100% pure Jason...unfortunately for me.