ChargeOver - ChargeOver automates recurring billing, subscription billing, & payment acceptance. A SaaS that integrates with Xero, Quickbooks, & has an API. Free trial.
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.
fusebill - Fusebill is the leading recurring billing and subscription management software for businesses. Try Fusebill for subscription billing made easy!
Elorus - A super-easy online invoicing & time tracking software
Street Invoice - Mobile Invoicing for Android, Blackberry and Windows PC
Cadebill - Integrated telecom billing, BSS OSS, and convergent charging