Simple Invoices - Never Wait For Clients To Pay You Again!
InvoicePlane - Free and open source invoicing application to host on your own server.
Express Invoice - Express Invoice is the easiest system for creating and tracking quotes, orders and invoices for your business. Use web access to view your data from anywhere.
Invoice Ninja - Invoice Ninja is a free, open-source solution for invoicing and billing customers.
FreshBooks - The ideal accounting software for small business owners.
Crater - Free Open-Source Invoice app for freelancers & small businesses.