InvoiceNinja

InvoiceNinja is an incredibly versatile invoicing system built on, you guessed it, open source. Accessible through any browser this project can be run from your own dedicated server at no cost and features every tool or adjustment you could think of. InvoiceNinja does however offer a hosted alternative for small business for free forever (limited customisability) and affordable pricing options for medium to large business'.

To find out more about this invoicing utility and their hosting options you can visit their commercial website here and organisation website here.