My favorite are the Gepe Card Safe Extreme watertight cases. The each hold four cards, whether CF, SD or almost any other type of media currently in use. I have several of them.
Not inexpensive (about $25-30), but about the most secure storage you can find. Most importantly, for those of us who work around and on the water, they are well-sealed from the elements. And, they float (though I've not yet had the occasion to need this feature)!