This is an online 3D Minecraft map viewer and dungeon finder that requires WebGL (new Firefox/Chrome).
It looks like this:
(Google translate of description in case Baidu will pick it up) minecraft地图浏览器地牢发现者 (minecraft dì tú liú lǎn qì dì láo fā xiàn zhě)
To determine Minecraft coordinates, hit F3 in Minecraft. Enter x and z coordinates rounded to whole numbers below. Filename will update automatically.
x: y: z:
Press the View button and on the next screen you will select the .mca file from your local machine under %appdata%/minecraft/saves/worldname/region/.