is young; its developer hasn't yet christened it with a "1.0" name. But it could save you money versus a paid tool; it's not just free to download, it's an open-source application, so other developers are free to make improvements and add features.
You'll install Bamboo Invoice on your PHP-and-MySQL compatible server, instead of trusting another company to hold your data. Since it's an online tool, you can log in from any PC to make updates, even if you're working offsite.
Once inside, you can create invoices, mark payments, add notes, and more. (Poke around the developer's for a sample.) Invoices can be cleanly formatted PDFs, showing moderate customization, such as your company logo. Simple reporting tools show your income and billings, and client profiles let you quickly pick a destination, instead of having to enter contact info more than once.
Multiple account options also let different employees create their own invoices. However, once you need that level of access, you might want to consider a more robust, paid invoicing tool.
Bamboo Invoice could match independent contractors and small businesses that are trying to save a few dollars. And since it's accessible online, it could be handier than a client-side tool, where you have to interact from a single PC. You could save in the initial cost, plus gain value in its accessibility.