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.

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Source code:
seantisinvoice on the repoze blog: