Billing Boss is definitely free, and you can currently create unlimited invoices for unlimited customers. Some other features we offer include:
-Multi-currency support
-Track all payments for better cash flow management
-E-mail support (send invoices directly to clients) and full PDF support (send attached PDFs to clients automatically)
-Accountant/bookkeeper access to all invoices, and track payments
-Recent launch of mobile payment tool (http://www.paymentboss.com) which allows you to go beyond creating invoices through your phone, but also collect payments while you’re on the road. This is completely integrated with Billing Boss, so you can track all your invoices and payments in Billing Boss.
-Mobile payment tool allows you to create a database of customer information which you can export to other applications. Useful for marketing campaigns, or when you need to reference a customer’s contact info while you’re on the road.

Full Disclosure: This author has been compensated by Sage.