I used furrtek's JavaScript code from the wiki (found at MediaWiki:Common.js).
The program supports standard NGFX .ngpal files, as well as a different format (.neopal) that allows for storing up to 256 named palette sets in a file. The program also allows for importing the palette from PNG files (preferred format is 4BPP/16 color; 8BPP/256 color is available, but only one 8BPP PNG file can be imported at a time.) Palettes can also be exported to "assembler ready" format (i.e. text based with labels), both single and ranged sets.
Source code is available under the Unlicense; download available on the Releases page or on my website.
