If you're on a diet, there's probably no better snack than seeds - the type that are still in their shell. Sunflower, pumpkin and watermelon seeds all take a bit of time to crack open with your teeth. Even if you snack incessantly on the edible kernels, you won't be consuming all that much - at least not if you compare that with mindlessly eating from a bag of potato chips or dipping your hand into a cookie jar. And if you're the type who eats the whole seed - shell and all - you can take comfort in the fact that you're getting a good dose of roughage.
Of course, if you are on a diet, you should probably avoid the seeds that are already hulled - they're a lot easier to eat, so you'll consume them a lot faster, and seeds tend to contain a fairly high dose of fat for something so small (many types of seeds are pressed for their oil). You should also avoid salted seeds - even if you're throwing away the shells, you'll still ingest a small dose of salt every time you crack one open.
If you're not dieting, though, salted and/or flavoured shelled seeds are much more delicious and satisfying than those still in their husks. Colourful candy- and chocolate-covered sunflower seeds are especially appealing; they look like miniature M&Ms (the colours are the same). I also like to scatter shelled and lightly salted pumpkin seeds (along with candied orange rind) over tempered chocolate when I make chocolate bark.