GnuCash - A personal and small-business financial-accounting software, licensed under GNU/GPL and available for Linux, Windows, Mac OS X, BSD, and Solaris.
Billdogg - Beautifully simple invoicing for freelancers
Xero - Xero is online accounting for your small business.
Free Invoice Generator - A free invoice generator for startups
CheckStream - CheckStream check printing software
Bookipi Invoice App - A minimal, beautiful invoice app for freelancers