This doesn't really classify as a DIY, as it's so simple! But I was really pleased with how it turned out, so I thought I'd share anyway, even if it is pretty self explanatory, they might inspire you to organise your fridge door!
You will need:
Wooden clothes pegs,
Letter stamps and an ink pad (you could handwrite your labels if you prefer),
Magnets - I used self adhesive magnetic tape, cut into strips, it's thin, therefore the pegs sit pretty flush to the fridge.
I'm thinking of making one for each of my family members, so we can write lists and notes for each other.