This may be a well-known trick to some, but it was new to us so we thought we'd share. In the past we've avoided ammonia in our cleaning routine because the smell seemed too strong. But with that powerful smell apparently comes a cleaning power from the fumes alone...
In a cold oven, place a glass dish with half a cup of full-strength ammonia. Close the oven and let it stand overnight. The next day the grime on the oven walls should be loosened sufficiently to be easily wiped away with a damp sponge.
As an added bonus, a Seattle P-I reader (we originally spotted this trick in this article in the P-I) suggests pouring the leftover ammonia into a garbage bag with the oven racks, tying it up and letting the bundle sit overnight in its turn. The oven racks will be similarly wipeable in the morning.
Edit: In the method mentioned above, we forgot to add that the bags should be left outside or in a garage overnight, not inside the home.