What is a good pre-workout snack while you are on a low calorie diet?

A bit o'carbs. Choose something like a granola bar to provide your body with some carbs for the workout. Grain cereals like grape nuts, a whole grain bagel or a banana are other good choices. Not a large amount though right before your workout because it will be sloshing around in your stomach and also it will draw too much blood to your gut rather than to your muscles for the workout.
Almonds... A few almonds are a great pre-work out snack if you are going low-cal. You may also want to try some cooked egg whites; and make sure you are well hydrated (water is my beverage of choice). Good luck!