Aaron's salmon recipe (coming to my table soon!) made me think I haven't made Mark Bittman's ponzu lately and I should -- it's fantastic! I googled and didn't immediately find it (except on the NYT site for $$$) so I fished out my yellowed clipping and here it is. You can get the konbu and bonito at an Asian grocery of course and also at Whole Foods (for more).
Minimalist Ponzu (Mark Bittman, NYT)
Ingredients as published, directions paraphrased
2/3 cup fresh lemon juice, more to taste
1/3 cup fresh lime juice, more to taste
1/4 cup rice vinegar
1 cup good-quality soy sauce
1/4 cup mirin (or 1/4 cup sake and 1 tablespoon sugar)
1 3-inch piece kelp (konbu)
1/2 cup (about 1/4 ounce) dried bonito flakes
Mix everything. Wait minimum 120 minutes, up to 12 hours. Strain. Adjust citrus. Keep in fridge.