It is a custom to bake a schlissel challah (key challah) for the first Shabbos after Passover. There are different traditions, with some baking their house key (wrapped in foil) into the challah, and some baking an image of a key into the top of the challah.
Another option is baking the challah in the shape of a key.
For my schlissel challah, I used this recipe, scaled up to use 5 lbs. of flour.
Last year Imamother polled its readers about this custom and found that most people favored baking a key into the challah.
I had thought this was something only done at home, but, apparently, you can also buy schlissel challah . . .
For more information on schlissel challah:
The Kosher Channel has this video (and an article) showing how to shape different schlissel challahs:
Cook Kosher has a shaping technique here.
There is a 2013 article in Hamodia about schlissel challah, with shaping illustration and a recipe. (There is also a 2014 article by Tamar Ansh, but it isn’t online yet.)
UPDATE: Another interesting article, “Schlissel Challah–An Analysis,” by Rabbi Yair Hoffman (5Towns Jewish Times, April 5, 2013).