Invoice Ninja - Invoice Ninja is a free, open-source solution for invoicing and billing customers.
TopTracker - Beautiful, free time tracking
Harvest - Simple time tracking, fast online invoicing, and powerful reporting software. Simplify employee timesheets and billing. Get started for free.
timeBuzzer - timeBuzzer enables freelancers and project-driven teams to track their time at the touch of a button, understand where their time goes and create reports with just a few clicks.
GnuCash - A personal and small-business financial-accounting software, licensed under GNU/GPL and available for Linux, Windows, Mac OS X, BSD, and Solaris.
Cattr - Cattr is a free open-source self-hosted time tracker with desktop clients for windows, mac, linux.