Onigiri

4 cups of freshly cooked Japanese-style rice
2 sheets of nori seaweed, cut into 2 inch wide strips
Salt

Traditional fillings:
Pickled plum (umeboshi), bonito flakes just moistened with soy sauce (okaka), bonito flakes mixed with pickled plum (umekaka), flaked cooked salted salmon (shiozake), cooked salty cod roe (tarako), chopped up pickles (tsukemono), and tsukudani, various tidbits - bonito cubes, tiny clams, etc. - cooked and preserved in a strong soy-sugar-sauce.

Some non-traditional fillings:
Ground meat (pork or beef or a mixture), cooked with grated or chopped ginger, then flavored with soy sauce, some red pepper flakes, sake or mirin, and sugar. It should be quite dry. Curry flavored ground meat mixture works surprisingly well too.
Canned tuna, well drained and flaked, flavored with a bit of soy sauce and/or salt to taste.
Chopped up western style pickles.

The key to making good onigiri is to have freshly cooked, hot rice.
Wet your hands with cold water, and sprinkle them with salt. Take 1/4th of the rice and place on one hand. Make a dent in the middle of the rice with your other hand. Put in about 1 tsp or so worth of filling in the dent. Working rapidly, wrap the rice around the filling, and form into a ball. To make the traditional triangular shape, cup your hand sharply to form each corner, and keep turning it until you are satisfied with its shape.

Wrap the rice ball with 1-2 strips of nori seaweed. Repeat for the rest of the rice.