BillGrid.com - BillGrid is an easy to use online invoicing, billing and expense management system that will save you time with tracking clients projects, expenses, invoices and payments.

FusionInvoice - Self hosted invoicing for freelancers and small businesses

ChargeOver - ChargeOver automates recurring billing, subscription billing, & payment acceptance. A SaaS that integrates with Xero, Quickbooks, & has an API. Free trial.

Invoice Ninja - Invoice Ninja is a free, open-source solution for invoicing and billing customers.

Elorus - A super-easy online invoicing & time tracking software

Zervant - Easy invoicing, expense management, reporting, and customer feedback.