I have been using Zoho Invoice for a few years now, and I really like it.
It takes away the tedious work of manually creating invoices, calculating tax, and keeping track of invoice numbers.
It also has really great views to display which invoices are overdue, paid, etc.
At the end of the year, you can pull up really nice reports detailing your income, revenue, expenses etc.
Creating invoices is a breeze once you have the program configured. It allows you to spend more time working and less time creating invoices.
You can email copies of the invoice as a PDF attachment and also send out reminders directly from its web interface.
Zoho Invoice comes in 4 price tiers: