I came back from Japan with numerous delectable food purchases. The first thing I brought was "yuzukosho". I was served this green paste as a condiment with nabe, a dish where you cook what you like in a pot on the table. It's a combination of the peel of the citrus fruit yuzu and kosho, a kind of chile pepper. I also had it with sashimi. Imagine the taste of limes and chiles with just a pinch of salt. Irresistible, right? So is yuzukosho! It is very common in the South of Japan, but a more recent addition to the table in places like Tokyo.
Another purchase I made was wasabi in a tube, not the pasty fake wasabi that is really just dry mustard colored to look like wasabi, but real wasabi. Real wasabi actually tastes like horseradish, not mustard. It's bright and hot but has plenty of flavor, not just heat. If you can find the fresh stuff, keep in mind the Japanese rhizome will taste a bit different from the American rhizome. It also needs to be grated on a sharkskin grater which results in a characteristically creamy texture. Even in Japan fresh wasabi was expensive. I usually request it at sushi bars and don't mind paying a little extra for it.
The other "hot stuff" I received at the Rise of Asia Worlds of Flavor conference last week. It's sansyo pepper. The light green powder is made from the ground up leaves of the prickly ash tree, the same tree that produces Szechuan peppercorns. It's a funny pepper, at first it just tastes herbal and slightly lemony, but after a few moments you get a tingly sensation in your mouth. It's not really hot, but kind of prickling; I can't think of another pepper with this effect.
Hot spices, herbs and pastes, are used in Japan, as are citrus flavors, to balance out greasiness or fat. Try using them on steak, fatty tuna or anywhere you'd like some heat. They each have tremendous flavor beyond the kick and are not that hot if used in moderation.