siwapp is a Web application meant to help manage and create invoices in a simple, straightforward way. It can manage invoices with different serial numbers (as rectification invoices), provide printable PDF versions of the invoices with a professional look & feel, save drafts of your invoices and continue editing them later, and automatically generate invoices on a periodic basis.
MacFreelance is an application for invoicing, billing and managing clients. It generates professional invoices, monitors time spent on projects, tracks mileage and payments, performs tax calculations, and generate detailed reports. It supports multiple companies to bill from, fixed or timed charges and billing in hours, minutes, days, weeks, or months, project quotes, global currency support and tax calculations, and integration with Address Book, iCal, and Spotlight.
MEVEO is a billing system for mediation, rating, invoicing, payment, dunning, and composition. It is adapted to the following situations: batch processing of thousands of invoices per month (or much more), complex price plans, real time rating, and re-rating, automatic provisionning through an API, multi-format CDRs, and pre/post-invoicing reports.