Use your Arduino as a direct USB to serial converter! Tested on an Arduino Duemilanove. Probably 3.3V TTL but worked for communicating with my supposedly 5V TTL HerkuleX servo at 115200 baud.

ArduinoSerialBypass.ino

/*
 * Arduino Serial Bypass - use an Arduino as a dumb USB 2 Serial Converter
 *
 * This code makes the Arduino not interfere with pins 0 and 1
 * which are connected to RX and TX on the FTDI chip. This allows
 * the data coming from the FTDI USB 2 Serial chip to flow directly
 * to another device. Since RX and TX are labeled from the Arduino's
 * point of view, don't cross the wires, but plug the device's 
 * RX wire into the RX pin 0 and the TX wire into the TX pin 0.
 *
 * Because this is a direct hardware connection, high baud rates
 * like 115200 are not a problem like they are with SoftwareSerial.
 *
 * Credit goes to "ihsan". His original website is down at the time
 * of writing, but I found the text archived on:
 * http://web.archive.org/web/20101027200300/http://students.sabanciuniv.edu/kehribar/?p=19
 */
void setup()
{
  pinMode(0,INPUT);
  pinMode(1,INPUT);
}
void loop()
{

}

Really works great and much more stable with esp8266, thanks for sharing!

Tested on this a Witty cloud ESP8266 board, works like a charm:

http://www.cnx-software.com/2015/12/14/3-compact-esp8266-board-includes-rgd-led-photo-resistor-buttons-and-a-usb-to-ttl-interface/

Nothing in setup() or loop() also silences the MCU, and is exactly the same.

You can also achieve this by shorting the RESET pin if you have a jumper wire handy.