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.
Xero - Xero is online accounting for your small business.
ChargeOver - ChargeOver automates recurring billing, subscription billing, & payment acceptance. A SaaS that integrates with Xero, Quickbooks, & has an API. Free trial.
Wave Financial - Award-winning financial software designed for entrepreneurs.
Elorus - A super-easy online invoicing & time tracking software
QuickBooks Online - QuickBooks Online is a Cloud Accounting for small business that can be used on iPad, mobile phone or computer to do invoicing, billing, run reports, or even pay employees online.