Arduino enthusiast Burak has created a wonderful LED clock that uses three rings of individually-addressed LEDs along with the usual numbers to create what could be classed as a work of art. There is also temperature and humidity display with the time. It's also good to see him using a real-time clock IC for accuracy instead of using internal Arduino functions.
Furthermore, you can download all the design files to make your own PCB and rendition of the clock. So to get started, head over to the project page here. And we're on twitter and Google+, so follow us for news and product updates as well.
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.