In it's current form it is a simple NBT to JSON transpiler, but in the future it will be able to edit NBT files in the browser. Simply drag a NBT file (gzipped or not, big-endian only for now) over the yellow-ish side of the page and drop it. The NBT file is that decompressed (if necessary) and coverted to well formated JSON.
Hosted Version: https://cubeengine.github.io/WebNBT