My go-to is fruit most of the time, or a low calorie cracker (rice crackers, for example) with something on top (hummus, cottage cheese, cottage cheese and tomato, peanut butter spread very thinly, Vegemite/ Marmite, etc), or celery (either plain or with something on top). Reduced fat yoghurt is also good. Sometimes I eat an entire punnet of cherry tomatoes while I'm studying (and do very little damage to the calorie allowance that way).
Depending on why you're snacking, also look at your liquid intake. Sometimes I just need flavour, rather than food, and so go for a glass of water with a splash of lemon juice in it (which is usually 5-10 calories, whereas most snacks will be at least 50).