Harvest - Simple time tracking, fast online invoicing, and powerful reporting software. Simplify employee timesheets and billing. Get started for free.
InvoicePlane - Free and open source invoicing application to host on your own server.
FreshBooks - The ideal accounting software for small business owners.
Billdogg - Beautifully simple invoicing for freelancers
Toggl - Toggl is an online time tracking tool. It features 1-click time tracking and helps you see where your time goes. Free and paid versions are available.
Zoho Invoice - Zoho Invoice is an easy invoice software meant for small businesses and freelancers.