Some good substitutes for chili powder include paprika, crushed red pepper flakes, cayenne powder and chipotle powder. Some people also use hot sauce as a substitute.
Paprika has a base of chili pepper, and some varieties contain more heat than others, such as cayenne pepper. If a cook has crushed red pepper flakes in the kitchen, he can grind them even further to create a seasoning similar to chili powder. If using chipotle powder as a substitute, the overall taste of the dish ends up smokier. If substituting cayenne pepper, the dish gets more heat than regular chili powder. In liquid dishes, using hot sauce gives similar results to using chili powder.