ChargeOver - ChargeOver automates recurring billing, subscription billing, & payment acceptance. A SaaS that integrates with Xero, Quickbooks, & has an API. Free trial.
Freelancer Stack - Curated directory of tools used by 10,000+ freelancers
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.
The Freelancer Toolkit - A collection of essential tools and services for freelancers
fusebill - Fusebill is the leading recurring billing and subscription management software for businesses. Try Fusebill for subscription billing made easy!
PlanArty - Time management solution dedicated to freelancers and small business owners