Invoice Ninja - Invoice Ninja is a free, open-source solution for invoicing and billing customers.
Envoice - Send invoices and get paid on Paypal, Payoneer, Stripe and Square.
Invoiced - Invoiced is simple invoicing for freelancers and small businesses.
InvoiceNeko - An open-source and free invoicing system
Free Invoice Generator - A free invoice generator for startups
Billdogg - Beautifully simple invoicing for freelancers