1. You could do this with MapReduce:
   mr = db.runCommand({
   "mapreduce" : "my_collection",
   "map" : function() {
   for (var key in this) { emit(key, null); }
   },
   "reduce" : function(key, stuff) { return null; },
   "out": "my_collection" + "_keys"
   })

Then run distinct on the resulting collection so as to find all the keys:
db[mr.result].distinct("_id")

["foo", "bar", "baz", "_id", ...]

2. Try this tool: https://github.com/variety/variety.git