Buy a Kitchen Scale

Slate weighs in. In all honesty, it's only a critical tool for baking, but that would change if more recipes were done by weight. Any marinade/sauce/vinaigrette where you're adding a ton of different ingredients in succession would be a lot faster and easier if you had weights... but alas nobody writes recipes that way.

EDIT: This is one of those posts that disappeared last week during the Blogger crash and has apparently been recovered.