Official U.S. time
Free service provided by the National Institute of Standards and Technology (NIST), and its military counterpart, the U. S. Naval Observatory (USNO).
http://www.time.gov/

The Perpetual Calendar - A helpful Tool to Postal Historians
Unlike the subtitle suggests a valuable resource for anyone interested in calendar history, and especially the use of the Julian and Gregorian calendars in various countries.
http://www.norbyhus.dk/calendar.html

World Timezones maps with current time
Very nice site offering visual presentation of the various time zone around the world. You'll also find a list of over 300 common time zone abbreviations with their description and UTC offset.
http://www.worldtimezone.com/