My sister called me the other day needing to know what spice she should substitute for cumin, and while nothing tastes quite like cumin - a little nutty, a bit bitter, and just a touch of peppery heat - a good alternative is chili powder. Why? Most chili powders contain cumin, so they are a good option. But, if you are allergic to cumin or not a fan of chili powder, you can also try ground coriander or caraway seeds.
For best results, regardless of which substitute you use, start with half as much as the recipe calls for, then adjust upwards to suit your taste.