Open WiFi settings on raspberry Pi

If you are trying to connect to an open wifi router with your Pi, change the following settings in
/etc/network/interfaces. These settings worked with Raspbian 2012-09-18-wheezy-raspian.

# my wifi device  auto wlan0  iface wlan0 inet dhcp wireless-essid [ESSID]   # your router SSID wireless-mode [MODE]  # usually mode is "managed"