seantis::invoice: Open Source Invoicing
seantisinvoice is a small web based invoicing and payment tracking solution for freelancers and small companies. It's designed to keep your processes as easy as possible. seantisinvoice is not a full scale customer relationship management (CRM), instead focusing on the core features.
To use the words of repoze.bfg: seantisinvoice is a "pay only for what you eat" invoicing solution.
For more informationen: http://invoice.seantis.ch