At startup, pins are configured as INPUT.
GPIO0-GPIO15 can be INPUT, OUTPUT, or INPUT_PULLUP.
GPIO16 can be INPUT, OUTPUT, or INPUT_PULLDOWN_16. It is also XPD for deepSleep() (Perhaps via a small capacitor.)
Note that GPIO6-GPIO11 are typically used to interface with the flash memory ICs on most esp8266 modules, so these pins should not generally be used.
From http://www.esp8266.com/wiki/doku.php?id=esp8266_gpio_pin_allocations