1. Julienne Peeler
Key for making quick work of turning things like carrots, beets, apples, jicama and more into matchstick-size bites. You can even use it to turn zucchini into "noodles." They're around $5 and I don't know how I ever lived without it.
I've talked before about the importance of texture in your salads - the julienne peeler and hand-held mandolin (below) will help you create super yummy salads and healthy dishes that you can't stop eating.
2. Hand-Held Mandolin Slicer
Just like the julienne peeler, the hand-held mandolin is key for shaving things like onions and other produce super thin and uniform. It's excellent for turning a sweet potato into chips, slicing cucumbers for quick pickles and tons more.
You can get a high-quality hand-held mandolin for around $20 and they're safer, easier to use, and easier to clean than their larger counterparts.
(Note: Mandolins are very sharp! Keep your fingers away and be very mindful when using one. My husband's been to the emergency room twice in the last 5 years after losing to a countertop mandolin.)
3. Citrus Press
I don't like a lot of gadgets in the kitchen and like to keep my drawers clutter-free. A citrus press always seemed unnecessary until I started using one; they really do double the amount of juice you get out of your lemons and limes. And when you're buying organic you want to get every last drop of juice you can.
Cut your citrus in half, put the cut side down (opposite the shape of the press) and squeeze away - you'll save a ton of money in the long run but getting the most out of your citrus. You can get a high-quality stainless steel press for around $20.
4. Glass Storage Containers
Plastic can leach nasty chemicals into your food; always use glass containers when you can. Plus, they last longer and will save you money in the long run. I also like glass containers because you can easily see what's in them.
Look for tempered glass with freezer-safe lids (like Pyrex) to store leftovers. I also keep a bunch of different sizes of mason jars around to store things like seeds and nuts in the refrigerator and to quickly shake up my own salad dressings.
5. Very Sharp Chef's Knife
If you have one really good piece of kitchen equipment, it needs to be a very sharp chef's knife. Look for one that's around 7"-8"; you'll be able to use it for just about any knife-related task in your kitchen. Stop trying to chop vegetables with a little paring knife - a chef's knife is the ultimate kitchen tool.
If you already have one, take it to a kitchen store and have it sharpened about once per year; you're less likely to cut yourself with a sharp knife, and a sharp knife makes preparing food so much easier and enjoyable. Keep it sharp by only using it for food (stop opening packages with it) and keep it out of the dishwasher (the spinning water hitting the blade dulls it over time). Prices vary quite a bit - buy the best one you can afford.