Hoisin sauce is commonly used in Chinese cooking, although it's used more in American-Chinese dishes than in traditional Chinese cuisine. Sometimes described as Asian barbecue sauce, it's sweet and sticky, yet also savory and slightly smoky, with complex flavors coming from Oriental spices. If you're looking for a substitute, nothing will replicate the flavor of this sauce exactly, but a few ingredients may come close.
Chee Hau Sauce
Like hoisin sauce, chee hau sauce -- also known as brown bean sauce -- is primarily made from soybeans, with some garlic, thus giving the two similar flavors. The main difference is that chee hau sauce is heavier on the chili peppers, so it's much spicier than hoisin sauce. Use it in the same quantities as you would hoisin sauce, unless you're worried about the extra spiciness, in which case you should use it more sparingly.
Barbecue Sauce and Five-Spice
Hoisin sauce has a lot in common with barbecue sauce. They both have sweet and savory notes, with a rich, garlicky flavor and a slight smokiness. What traditional barbecue sauce lacks, however, is any oriental spices. By adding some Chinese five-spice to store-bought or homemade barbecue sauce, you'll make it taste similar to hoisin sauce. Use the same amount of barbecue sauce as you would hoisin, but add a generous pinch of five-spice powder.
Sweet Bean Sauce
As with chee hau and hoisin sauce, sweet bean sauce is primarily made from soybeans. Sweet bean sauce is made from sweetened fermented soybeans, which gives it a similar sweetness to hoisin sauce plus a complexity of flavor. Since it's a good deal more spicy, you may want to use slightly less of this than you would hoisin sauce.
While apple butter doesn't have too close a flavor profile to hoisin sauce, it's sweet-tasting and has a similar dense, sticky texture. Mixed with some salt or soy sauce, some garlic powder or finely minced garlic, and even a pinch of five-spice powder, it yields fairly similar results to using hoisin sauce. It will still have a marked apple flavor to it, so use a smaller amount than you would normally use of hoisin sauce.