World Time – This free program shows you the time anywhere in the world, including phases of the moon and times of sunrise and set. It allows for daylight saving and will show you the daylight saving rules in use anywhere in the world. It can also chime the hours, set your computer's clock to the nearest second, has a stopwatch and alarms... sorry, it doesn't make coffee.
Dimension 4 – Free software that specialises in setting your computer's internal clock from an atomic clock (to the nearest 1/20th of a second). If you want to keep your computer clock accurate this software will do it in a totally inobtrusive manner whenever you connect to the Internet.
If you are not a Windows user, consult this software list.