Leave it to the Europeans for coming up with the tradition of a house warming. The truth is though, that many cultures have their own variations of a house warming. The original items of a broom to "sweep away evil", and a "pinch" of salt at the entrance of each door to ward off evil and bring good luck, and a slice of bread so the new owners will never go hungry are more or less universal but depending on where you live, other items have their own significance when it comes to a house warming.
For example, give the new home owners a candle so they will always have light, wine so they never go thirsty, olive oil to bring good health, honey so the couple's life will always be sweet and a plant which represents a long life. There's actually a poem - supposedly of Italian origin -- that speaks of several of these traditional items:
Bread so your larder will always be full
Salt to give it savour
Honey for a sweet life in your new home
Here's another version from Germany:
Bread so you'll never go hungry,
Salts so you'll have good luck
Wine so you'll never go thirsty
The whole bread-salt-honey as a house warming motif is linked with many countries. Speak to an Italian and they'll tell you Italy started the tradition of house warming. Talk to a Russian and they insist it was Russia who initiated gifts of bread and salt. Some people are convinced that the concept of "house warming" reflects when visitors brought hot coals or embers to a new house in order to keep it warm and hence the term. At this point in time who exactly stated these house warming trends is a moot point. The fact that the tradition is observed and respected is much more important.
In the Orient for example, where rice was more than a fast food item to be eaten with chopsticks, it was given to newlyweds as a sign of fertility. That's why even today we still throw rice at weddings.
Leave it to Americans to be much more realistic when it comes to a house warming party. They seem to have canned most of the traditional bric-a-brac in favor of useful items. Moving into a new home? Here's a blender. New apartment? A throw rug. Certainly these items are probably more appreciated than bread and salt. And really, what are you going to do if you invite 20 guests to a housewarming party and they all bring the same thing?
Still, you can't go wrong if you try to incorporate some sense of tradition in your housewarming gift. An owl is a big item in Italy. It is supposed to bring good luck and ward off evil at the same time. Other cultures let a cat enter the house first. Well obviously you're not going to bring live animals to a house warming, but statues or said animals may be appreciated, especially if you explain the history behind them.
Another tradition - again courtesy of Europe - is to hang a horseshoe over the door for good luck. And if you do hang a horseshoe over the door make sure it's securely fastened. Nothing worse than a symbol of good luck falling off and whacking the happy couple in the head. Bad way to stat off a new life!
No matter what gift you give, it's the thought that counts. And if worse comes to worse just give money. Not as flashy or impressive as a gift - but I guarantee it will be appreciated just the same.