With simple hacks the Arduino board (with USB programming interface) can be used as a USB to serial conversion device. There are two methods to do this.

I have been playing with wCK Servo from RoboBuilder for a while now. This servo packs quite a punch:

Bluetooth modules come really cheap these days, affordable alternatives to Xbee. I got my hands on a BTM-182 from Rayson. Like most Bluetooth modules, it comes with Serial Port Profile. SPP is basically a serial port emulator; in a nut shell - once its paired with a computer, you get a virtual COM port to play with.

Rugged Circuits make Ruggeduino, Arduino on steroids - with overvoltage protection, overcurrent protection, ESD protection. Great stuff for people like me, "oops! I made a boo-boo". Hat tip to Rugged Circuits for showing us 10 ways to destroy an Arduino AND methods to prevent them. Read all about it.