App that tells you that joe owes you £20 (split bills / holidays between friends). See it in action at http://billwaldo.com
Rails 3.1 (beta) MongoDB jQuery Deployed on Heroku
Only share bills with people you trust. There's no administrator so if one of your friends is feeling evil they can delete / lie about expenses.
Enhance through layout, images and ajax.
Email people when they are added to a new bill or when a bill moves to the payment phase.
Payment groups - so a couple's payments are combined.
Currency conversions and allow cross-currency expenses in a single bill.
Audit trail - to keep the convenience of the admin-free set-up but cancel out the impact of careless / malicious friends.
See the issues list for more details.