Chris Coyier created this wonderful editable, printable invoice a while ago, along with work by Vinh Pham. I needed an invoice system for my business, so I began using his template. I made some tweaks to include my logo and contact information so I didn’t have to add it every time, and tweaked the title to include the date (I’m a nerd and like to file things by year-month-day). I also added a Discount field for those times when I feel like I should give someone a discount, the ability to remove it when they don’t.
I plan on adding a database or some other type of storage function to store post invoices/receipts—that should make it much more useful for a small business or sole proprietorship.