Since I don’t want to dedicate a $22 Arduino UNO [UPDATE: now $19] for every project (for space reasons, not price), I’ve started embedding a Atmel 328 chip into my projects. You can build your own Arduino UNO compatible for approximately $15 (buy parts in bulk, and make several), depending on the options you want to include. This DIY Arduino will be the foundation for a series of books we are writing based on the Arduino and general electronics.
The bare minimum needed to build is the following (click DIY Arduino link above to get parts):
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Qty. (1) ATMEGA328P (with Arduino Bootloader)
Qty. (1) 28 pin socket (optional, but highly recommended)
Qty. (2) 22pf ceramic capacitors
Qty. (2) 100nf (.1 uf) ceramic capacitors
Qty. (1) 16 mHz crystal
Qty. (1) 6 pin male header (for FTDI Programming Cable, see parts link above)
Optional:
Qty. (1) 150 ohm 1/4w resistor &
Qty. (1) Green LED (pin 13 indicator)
Qty. (1) PCB Momentary Tactile Push Button Switch (reset)
Power supply:
Qty. (1) LM7805 5v voltage regulator
Qty. (2) 10uf electrolytic caps
Qty. (1) Green LED
Qty. (1) 150 Ohm 1/4w resistor
Full instructions on assembly are located at http://www.instructables.com/id/The-Embedded-Arduino-building-an-Arduino-into-a-p/