Hey ,
My evening 100 is usually a couple biscuits, although I have to be very strict to make sure I'm not tempted to add a couple more!
The lowest one I have is
NICE (Tesco Value) - 40cal in each!
Some others I like are
Malted Milk (Tesco Value) - 46
and Maryland Low Fat Choc Chip Cookies - 50 each.
So you can afford to have a couple as a 100 cal snack and not feel bad!