InvoicePlane - Free and open source invoicing application to host on your own server.
Odoo - An all-integrated business app suite to unleash your growth potential.
Invoice Ninja - Invoice Ninja is a free, open-source solution for invoicing and billing customers.
Oracle NetSuite - NetSuite is the leading integrated cloud business software suite, including business accounting, ERP, CRM and ecommerce software.
Crater - Free Open-Source Invoice app for freelancers & small businesses.
Sage Intacct - Intacct is a cloud-based accounting and financial management system that is designed for the businesses of all levels.