Timeanddate.com is a sizeable website offering all sorts of resource related to dates and times. For the business user, it is probably most useful if you trade overseas - their easy use World Clock and Timezones sections are especially useful, particularly as they have also added a meetings function.
The Meeting Planner allows you to arrange a meeting with someone from a different timezone, by helping you to find the best time for everyone across the timezones.
Timeanddate.com also offer timers and a stopwatch, so if you need an app to help you keep track of how long you've spent on a task this could be a good place to go.
Another useful little tool is a calculator to help you work out how many working dates until a particular date. Good for working out how long you might have to wait to get paid sometimes!
There is so much detail in this website, it is difficult not to get carried away just browsing around. For example, their Astronomical Calculators help you to find out about sunrise and sunset, when the next eclipse will occur and whether you'll be able to see it where you are, as well as information on the cycle of the moon and a fascinating map of the world displaying where it is night and where it is day.
So do take a look at this website next time you have a few minutes, you'll find some useful apps and a fact or two with which to regale your colleagues!
Post Script: Since writing this we've been reliably informed that timeanddate.com is not very accessible for sight-impaired individuals. A more user-friendly site if you're using a screen reader is http://www.thetimenow.com.