Advanced Clock Plugin is a highly customizable replacement for the stock clock of the N900. Advanced Clock Plugin comes with different clock styles and offers many settings to further customize the look of the clock.
Furthermore Advanced Clock Plugin is designed such that custom clock styles can be added as well. To add custom clock styles basic Python knowledge is required. The most straight forward approach to add a new style is to use an existing style as basis, copy it to a new file and then start hacking around. More information about Advanced Clock Plugin can be found the corresponding talk.maemo.org thread: http://talk.maemo.org/showthread.php?t=67408
The source code can be downloaded from the Maemo garage site at: https://garage.maemo.org/projects/advclockplugin