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

flyceipts - A fresh look at digital receipts & other documents.

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.

Keep Your Friends Close - A free e-book about maximizing Customer Lifetime Value

fusebill - Fusebill is the leading recurring billing and subscription management software for businesses. Try Fusebill for subscription billing made easy!

Ramp Receipts - Create a receipts/invoice dashboard for Stripe