I just finished (the electronics side anyway) my arduino-based LED 7 segment clock
Code to follow when I have cleaned and commented it, but it's based on the LedControl library and the code / help here... http://forums.ladyada.net/viewtopic.php?t=4418
All lit up in a dark room...
Top view...
Back view...
It looks AWESOME at night. The digits are 1" high, very bright.
I use USB to power it, there is no communication over USB though. There is a serial port that I used for debugging, I don't use it any more. The two buttons are for setting the time, it increments the hours and minutes as longs as the button is held during start up. Time wise it loses less than a seconds over 24 hours, which is the longest period I've had my computer switched on.
The 7 segment displays are soldered to my own custom design and etched boards, nothing special, just to go from female headers to the correct segments. The rest of the board is plain stripboard, which was easy to use because each digit is multiplexed, all seg As connected etc, so I use a wire link to connect each segment to a corresponding copper strip.
It uses a 32kHz watch crystal instead of a normal 16Mhz crystal or resonator. It is based on the lilypad bootloader.
Next "upgrade" is to add a RTC with battery backup, so I don't have to set the time each time I turn it on.