I love when leftover cider starts to ferment in the fridge and gets all effervescent, and just a shade boozy. Yummy.
I have some apple-pear cider from seedling orchard (at Green City Market) that was really lovely when it was fresh, and these last couple of glasses that fermented are pretty darn nice too.
As a child I would love it when the cider would just start to get the sparkle in it.