Chameleon Clock is a very nice clock for Windows.
It can be minimized to the icon, so it allows you to control everything from your system tray. It is highly customizable and includes additional things, apart from the system tray clock, like a clock wallpaper and screensaver. It even includes a handy calendar that is shown when hovering the mouse over the tray clock.
You can set multiple alarms and record birthdays with reminders not to forget one ever again.
It includes several atomic clock servers to choose from and allows you to add specific time zones to show the exact time of the selected one. What is more, it includes a backup feature that enables users to schedule backups in order to save the stored information in case of system failure, virus attack, etc.
It includes numerous skins for your tray clock, but only one for your desktop wallpaper and screensaver. However, you can use your own skins or download more from developer's website.
Other features include quick reminders, countdown, and stopwatch. The program is very easy to use and has a nice and intuitive user interface.