• Sarbyx TrayClock

From author's site:

Sarbyx TrayClock is the replacement for standard Windows clock, it builds itself right into the taskbar and doesn't eat any desktop space. It includes numerous useful features that extend standard Windows clock functionality:
- to show or not current date or seconds;
- support 12 and 24 hours format;
- transparent background;
- possibility to use Winamp skins;
- 50 skins in the distribution;