For an Asian flair, I like to marinate extra firm tofu in a mixture of mirin, soy sauce, and grated ginger, thinned with a little water and boiled to caramelize the mirin. If I'm feeling crazy, I'll add a tablespoon or so of apricot or plum preserves.
For a spicy tofu dish, I'll marinate it in Tabasco or Texas Pete, salt and black pepper.
Really, the possibilities are endless. Anything you've ever heard of used on meat dishes can be adapted for tofu.
Before you marinate, though, make sure you drain and press the tofu.