One of the least fun parts of business is actually sending out invoices and chasing them. Lots of times, consultants and early stage companies don't have the capabilities (both human and software) to prepare professional style invoices.
Enter Blinksale, a company that allows you to track your invoices online as well, as send professional invoices straight to ones email box. You can even subscribe to the invoices using ical (mac) or RSS.
Quickbooks offers the service of emailing invoices and does it extremely well. However, there are tons of people who want to issue invoices who aren't set up with a professional bookkeeping package or don't want to make the monetary investment.