Multi-zone tax accounts (incl. different currencies)
Multi-currency
Flexible billing periods: Prepaid, Postpaid, Current
Stock & Inventory management
Allow tracking of where the particular item was purchased (think RMA's, returns … etc) - Manufacturing (combine one or more items into another item, correctly costing this item, including making provision for region specific intricacies relating to the cost of manufacturing and how its accounted for) - Ability to pull an item out of a manufactured product and sell it (you build 5 machines, but need to urgently pull the motor from one and repair one a client brought in) - Another thing that came up in a number of audit meetings is some large companies account for the cost for moving an inventory item from one warehouse to another, I've seen at least a handfull of companies that that need to see the cost of tracked items based on their original purchase value plus the cost of moving it
Online login for clients to manage services
Plugin based and flexible
Rich API
Invoice item grouping
Custom note + order number
Group items onto separate invoices
Assets and configurable depreciation
Provide an easy way to configure a type of asset, how its depreciated, and allow customization of edge cases.
I'd say the most interesting case I've dealt with is where an asset had its value re-evaluated during the course of depreciation, this was deferred to the auditors responsible for that audit to resolve .. but at some stage I need to fully wrap my head around it and the backend transactions involved.