Over the holidays my wife purchased a torch for me, and it's come in handy mainly for searing meat after a trip in the sous vide water bath. I've been experimenting with other ways to take advantage of this tool, and when my wife picked up some sushi grade salmon from work, we decided to make some nigiri.
With a few of them, I used the torch to sear the top layer of the salmon to give it a nice, smokey taste, just as many of the local sushi restaurants do around Boston. This works best on the salmon belly, which is the most fatty part of the salmon. If you're going to use the torch on food, be sure to aim it away from the food when first igniting it - otherwise, some of the gas may not fully burn away and you'll be left with an unpleasant taste on your food.