In its basic form, you just need the Atmega microcontroller, the crystal and 2 caps. A good idea is a 5V voltage regulator as well so you can use power sources bigger than 5V. The rest of the stuff on an Arduino Uno is not needed on a final project.
This board includes the ATMEGA328P-PU IC,
Skeleton Duino Features:
- The power terminals uses 2.54 pitch if you want to use screw terminals
- 5V regulator
- Power LED
- Status LED
- Dimensions: 45mm x 25mm
- ……uh, ok that it.