Freshbooks is an online service for automatically sending out either electronic or paper invoices to your customers with many addons and functionality that your customers will appreciate. Heres how it works:

Go to Freshbooks.com and sign up for the service. I will be upfront with you and say that this is not totally free but it is reasonably priced and your first 3 clients that you invoice are free. After that it is 19 dollars for 25 clients and 29 for a hundred clients. You can go all the way to 5000 clients serviced by this system. Once you have created your account, you can then setup your client list and begin creating quick invoices that can be delivered either electronically or have an actual paper invoice be sent through the mail. The paper invoices give you options to add your company logo and information to the printed material so it looks very professional and looks like it came from your business and not someone else. Customers then have the option to mail you the payment or log into the website and pay online. This is what sealed the deal for me. I hate getting checks in the mail and then having to go to the bank and make a deposit. They are also working on a Quickbooks plugin that will automatically update your quickbooks records to show if payment has been recieved. Pretty nice!

Freshbooks also has a few addons that you can integrate with your account to perform certain tasks. These include mobile applications for the iPhone and windows mobile devices that let you manage your account and track time spent working with clients. Desktop clients are also provided for Windows and Mac.