GWErp includes the following features:
Product Manager (inventory)
Multiple tax rates and currency
System backup / restore
Updates with a single click
In addition GWErp has different characteristics associated with billing will make your life much easier as a freelance:
Generation of e-invoice in pdf
Sending e-bill directly to the customer
Facilitates the flow of your income, your clients associated to web users can log on the frontend, your customers can access their bills anytime, from anywhere in the world where they have internet access and download your e-bill in pdf.
Also available in the Android Market.
Support it's not free, check: