Include a reference number for projects (in addition to a project name)
Having a reference number for projects is really helpful for identifying the job within big organisations that I invoice. The project name is often my own code-name to identify a project, but an additional ID (or purchase order no.) would be very helpful.
The ability to include this reference on invoices is good for allowing the finance / billing department to identify the budget to pay the invoice from, and also to allow for personal details to be excluded from the invoice (I produce reports on individuals)