The estimate/invoice/customer billing plugin you need

This plugin has single-handedly streamlined our customer estimate, invoicing, and payment processing in an easy to understand and set up interface. The free version is quite feature-rich, though we will be upgrading to the Business package shortly. The add-on list is both useful and plentiful to get some further value to the plugin. Well worth the time to invest in using the free version, and even more worth it to upgrade.