You can easily track outstanding invoices in Financial Cent's Account Receivable Aging Report. This report shows you how much and how long unpaid invoices are both from a wholistic perspective as well as on a per client basis.

You can either search for a specific client or filter by a client group to see the outstanding or overdue payments in both table and graph form.

Additionally, clicking on a dollar amount takes you to the "Invoices" page for that client so that you can quickly see which unpaid invoices and send reminders.