Ask HN: simple internal purchase order web app

I need a fairly simple purchase order system for tracking and approving our own outgoing expenses at work. Too much is just "happening" on its own but we don't need a big system to do it. I could probably write one relatively quickly but I'm sure there must be something simple, elegant and easy to use out there.

Basic needs are the ability to add suppliers, add a hierarchy of approvers and the ability to attach invoices etc to the POs when they eventually come in.