Billmonk definitely. It sends emails whenever a new bill is shared among users, you can shuffle debts when many people owe money to others in the same group. It supports receiving instructions via SMS emails (SMS sent to an email address), but I'm not sure if a regular email would work though.
Update: Billmonk regularly fails to work correctly (repeating bills haven't worked for months now) and I wouldn't recommend people to use it anymore until everything is fixed.