Since I wanted to debug my Arduino using my mobile phone (Samsung Galaxy S2) using the Arduino’s comfortable serial library, I bought a cheap USB-OTG adapter cable (male micro-USB-B to female USB-A) in order to connect the Arduino to my phone. The USB-A plug of the normal Arduino cable connecting an Arduino to the computer can be plugged into the above mentioned USB-A socket and again, the USB-B plug of that cable is connected to the phone.
After connecting it I had some difficulties to find a working app for my phone allowing to communicate with my Arduino using the serial interface. After some searching I found Slick USB 2 Serial Terminal which fulfilled its job fine for me. It is important to select “Prolific” mode in order to receive and send data from and to your Arduino. If your phone supports USB host mode, you can even use your phone to power the Arduino, so no external power supply is required.