1. Food Scale - Measuring cups offer a lot of precision, but when it comes to some types of items (frozen foods and meat in particular) nothing beats out a good, precise food scale.
2. Blender or food processor - too many uses to mention them all, but supplements tend to go down easier if they are blended with real food. Pureed fruits or veggies can be substituted for some fats in baking and cooking. You simply can't get things blended smoothly without one of these two things.
3. Rice Cooker - yes, it only does one thing but it does it so well. Cooking rice on an electric stove requires so much fussing and it's never quite done. If you don't like to spend a lot of time cooking, or just fancy really well made rice, it's worth $15 for an appliance you can use weekly or even daily for years.
4. Teflon - a whole set is expensive, but you can get by with one nicely constructed skillet for almost everything you want to do on the stove. Teflon coated tools are cheap and clean up easier too!
5. Steamer basket - for microwave or one that fits in your favorite pot, because at some point you will probably start eating fresh, seasonal produce. Why? It just tastes that much better and it tends to be cheaper too. Fresh, lightly steamed veggies are the most nutritious way to eat these items if you can't stomach pounds of raw broccoli.