nBill Lite is the free GPL-licensed edition of nBill. It allows you to create client and product records, and use these to quickly create and send professional looking invoices which can be paid online. It has a mobile-friendly interface, allowing you to create invoices on the go, and your invoices can incorporate QR codes, which allow your customers to pay by scanning the barcode with their mobile phone. You can also record income both from paid invoices, and from other sources, and view sales graphs showing how your income has changed over time.
It comes with a free payment gateway for Paypal, but other payment gateways are also available. If you are based in the UK, there is even a Direct Debit gateway available (using GoCardless), which allows your clients to set up a pre-authorisation, and allows you to take payment directly for any invoice you create for that client (it also has much lower processing fees than credit cards - just 1% per transaction).
The lite edition does not support recurring invoices, will not allow for products to be ordered online, and cannot be used to send quotes or reminders. For a full comparison of what is included in each edition of nBill, please see the feature comparison page on the main nBill website.