Account Payable. The offerings by K2 and BlueThread are by far the most compelling solutions I have seen to date around concrete application delivery for SharePoint. Video 1 of this series is an introductory video featuring Ron Douglas of K2 and Brian Partin of BlueThread. Videos 2 and 3 are actual customer ready demonstration videos showing the solutions use for Accounts Payable and New Account.
Bluethread K2 Video