Memrise is another great free flashcard learning program, one that allows you to add pictures and customize your flashcards. Both the NGSL and NAWL are up on Memrise in various forms. Here are two links to get you started:
The NGSL can be studied on Memrise in blocks of 100 words via this set.
The NAWL can be studied on Memrise in 6 blocks of words via this set.