Rogue Galaxy - Earth Key
The very first key you can get in Rogue Galaxy is the Earth Key and it can be found in the Chief's hut on Burkaqua. I feel as if this is in such an obvious location that it isn't obvious, usually I explore everything but somehow I missed both keys. Keep in mind that the next key follows a similar fate as this one meaning it can be found a future allies "home".
Also the next key you can acquire, the Star Key, requires the Earth Key to unlock. If you missed the key your first time on the planet worry not, I didn't get my keys until half way through the game so it isn't entirely gamebreaking if you miss it early on.