Answer:
   
   The nicest housewarning gift would be a mezuzah, which Jews affix to
   the doorpost. If you don't have a Judaica store near you, you can
   order one through www.judaica.com. Take a look at
   [5]http://www.judaism.com/mezuzah/ You'll also need to get a mezuzah
   parchment, which they have available.
   
   If that option doesn't work, consider a good Jewish book, or a gift
   certificate for a Judaica store. Both would allow the family to start
   a good Jewish library.
   
   Lastly, if neither of those pan out, consider something nice for the
   house. If the family keeps Kosher, avoid getting items used for food,
   as you don't know if they could be made Kosher. However, other items,
   such as picture frames, plants, etc. are lovely.
   
   Of course, the best gift is your good wishes and the knowledge that
   you are a friend. The handmade stuff is nice as well: knowing they'll
   be tired, offer to take them to dinner at a restaurant of their choice
   :-)