Adafruit Pro Trinket – 5V 16MHz

Here's some things you may have to consider when adapting Arduino sketches:

Pins #2 and #7 are not available (they are exclusively for USB)
The onboard 5V regulator can provide 150mA output, not 800mA out
You cannot plug shields directly into the Pro Trinket
There is no Serial-to-USB chip onboard. This is to keep the Pro Trinket small and inexpensive, you can use any FTDI cable to connect to the FTDI port for a Serial connection. The USB connection is for uploading new code only.
The bootloader on the Pro Trinket use 4KB of FLASH so the maximum sketch size is 28,672 bytes. The bootloader does not affect RAM usage.
Here's some handy specifications:

ATmega328P onboad chip in QFN package
16MHz clock rate, 28K FLASH available
USB bootloader with a nice LED indicator looks just like a USBtinyISP so you can program it with AVRdude and/or the Arduino IDE (with a few simple config modifications).
Also has headers for an FTDI port for reprogramming
Micro-USB jack for power and/or USB uploading, you can put it in a box or tape it up and use any USB cable for when you want to reprogram.
On-board 5.0V power regulator with 150mA output capability and ultra-low dropout. Up to 16V input, reverse-polarity protection, thermal and current-limit protection.
Power with either USB or external output (such as a battery) - it'll automatically switch over
On-board green power LED and red pin #13 LED
Reset button for entering the bootloader or restarting the program.
Works with 99% of existing Arduino sketches (anything that doesn't use more than 28K, and doesn't require pins #2 and #7)
Mounting holes!