A whole grapefruit... I looove grapefruit, and as a kid I was only ever allowed half of one, so eating the whole thing feels like an indulgence.

That's my favorite evening snack lately.
Coffee with half a packet of low-cal hot cocoa mixed in. For a real treat, throw in a peppermint hard candy (like one of those Starlight mints).
Lowfat yogurt with a little maple syrup mixed in. Probably my favorite food ever. I guess it's not especially low in calories.. but it is healthy.