There's an almost unlimited number of clock types you can make based around the Arduino platform, thanks to the ease of connecting real-time clock ICs and various output devices. One neat example of this has been demonstrated by Instructables member paulindallas who has built a neat word clock.
For the uninitiated, a word clock is just that - a clock that displays the time using words to make up a sentence in the same way that most people would say what time it is. Overall it's pretty simple, but provides a neat talking point and helps further the constructor's knowledge of all things Arduino.
The most important part of any clock project is the inclusion of an accurate real-time clock IC. Here at Freetronics we have the Maxim DS3232 real-time clock IC module:
Apart from keeping accurate time for years due to the temperature-controlled oscillator and having a tiny coin-cell for backup, it is very simple to connect to your Arduino project. A driver library allows your program to easily set or read the time and date. Perfect for clock projects, dataloggers or anything that needs to know the date and time. Furthermore it contains a digital thermometer and 236 bytes of non-volatile memory to store user settings and other data. For more information, check out the module page here.