Sort numbers by binary 1's count
J (11)

(\:+/"1@#:)


This is a function that takes a list:

     (\:+/"1@#:) 15342 28943 16375 3944 11746 825 32425 28436 21826 15752 19944
16375 15342 32425 28943 11746 28436 19944 3944 15752 825 21826


If you want to give it a name, it costs one extra character:

     f=:\:+/"1@#:
     f 15342 28943 16375 3944 11746 825 32425 28436 21826 15752 19944
16375 15342 32425 28943 11746 28436 19944 3944 15752 825 21826


Explanation:


\:: downwards sort on
+/: sum of
"1: each row of
#:: binary representation