The Clif "Kid" Z Bar is one of my most favorite snack bars. It is very healthy -- it is organic, whole grain, has fiber, and has no artificial ingredients. It is also quite yummy, and filling. It is a great snack. They come in an array of flavors (in order from my favorite to least favorite): Chocolate Chip, Honey Graham, Peanut Butter, Brownie, Blueberry, and Apple Cinnamon. They are super cheap (maybe because they target kids instead of dieters), and they are available for $.49/each at my local Whole Foods. They are also available at Trader Joes (last I saw they were $.69/each). Lastly, they are available online at the Yummy Diet Food Amazon Store. Each bar is a satisfying, yummy, and they are 2 WW points (except the peanut butter flavor, which is 3 WW points).
Shopping Tip: When buying bars at the supermarket, make sure you look at the actual ingredients (stay away from high fructose corn syrup, or partially hydrogenated ingredients) calories, amount of fat, amount of fiber, and the amount of protein. Good, yummy, bars for your diet are usually all natural, and less than 200 calories or so (although some are a bit higher in calories, and also in fat, because they contain nuts, which is okay in moderation).
Here are some links to a few of my other favorite low calorie bars: gnu foods, fruitabu, fiberful, luna tea cakes, attune probiotic bar, and figamajigs.