Surprised that this one hasn't been suggested already but a modern piggy-bank which automatically sorts all your loose change into denominations whilst at the same time keeping track of your finances on a display.
Needs to work with different currencies and be adaptable to allow for changes in coin size/weight over time. Bonus if it can dispense coins as well whilst keeping track of the remaining value of coins in the piggy-bank.
Could be adapted for 'charity bottle' type use, in which case the more 'entertainment value' given when inserting coins, the better.
