When I first got the thing, I had no idea how I would ever use it--I'm no medieval herbalist. But I've found that a mortar and pestle is the best way to get great spice flavor in any dish.
Ground spices lose their flavor much more quickly than whole ones; think of coffee made from ground beans that have been in the cabinet for weeks versus whole beans you grind right before brewing. Hand-grind a handful of whole cumin seeds to flavor beans or tacos and you'll get all kinds of tastes the jar of ground cumin you bought two years ago just doesn't contain. Ground mustard from a canister tastes like nothing, but smash a few mustard seeds in a mortar and pestle and you'll be greeted with wonderful smells and flavors for a vinaigrette or spice rub.
And what's more, whole spices, especially from the bulk bins at your local natural food store, are super cheap. I bought a tub of whole cumin for about $3 that's still three-quarters full after a year of weekly (or more) use. This doesn't involve very much time or effort, either. Grinding a tablespoon of spices takes maybe a minute and you don't need to be a bodybuilder, I promise.
My project for the summer? Grinding fresh pesto in my mortar and pestle, as in Chow.com's The Perfect Pesto video. It looks creamy and wonderful.
More Mortar and Pestle Recipes: