Snarl - Snarl is a notification system for Windows.
Billdogg - Beautifully simple invoicing for freelancers
Notifu - Notifu, a free open source pop-up balloon utility
Free Invoice Generator - A free invoice generator for startups
Notify OSD - Canonicals on-screen-display notification agent, implementing the freedesktop.
Bookipi Invoice App - A minimal, beautiful invoice app for freelancers