Unless you are grinding a very large quantity of spices at one time, the best tool for the job is the tool that was made to do the job: a peppermill.

I keep a peppermill just for grinding spices other than pepper. It's the smallest size that Perfex makes, and yes, I do recommend Perfex. Perfex grinders have worked so well for me over the years that I've never been tempted to try another kind, no matter how swoopy and chic the other kinds may be.

I have two that I use for grinding pepper, one in the kitchen, the other on the supper table; one for salt; and a small one for non-pepper spices.

I don't wash my spice grinder between uses, which means I probably get infinitesimal flavor contamination from one spice to another. So be it.

If you are grinding a large amount of spices at one time, a clean electric burr-type coffee mill is the tool of choice. I do wash out my coffee mill after using it because I use it so seldom that any residual spice fragments would become stale between uses. And I never use it for grinding coffee; Mark is the barista in our house, and he grinds his coffee in his own grinders.