If you do any sort of freelance work, you'll quickly run in to some logistical headaches. Namely, creating estimates, sending invoices and getting paid. You'll often have clients scattered all over the country, trying to coordinate client relationships & paperwork quickly turns in to a massive headache.  I ran in to the same issue, until I discovered Freshbooks.

Freshbooks is your one stop shop for all your freelance needs. You get to create lists of clients with contact information, easily send estimates to them & track their payments to you.  This is a service that a lot of websites provide, but what really impresses me with Freshbooks are the little things.

• Your clients can log in to your own branded (colors + logo) account center, where they can view their estimates and pay their invoices.
• Freshbooks integrates in to a dozen payment gateways, including Verisign, Authorize.net & Paypal.
• You can upload your print-quality logo, and Freshbooks wil snail mail your clients your invoices for you!
• Freshbooks also lets you track your expenses like hosting, which you can then rebill your clients for.
• Time tracking is included, which isn't unheard of, but Freshbooks gives you OSX and Vista widgets to help you track your time.
• Freshbooks even has a support system where your clients can open tickets, you can track the time on those tickets and then bill your clients for that time.

I could go on and on, but you get the idea.  Freshbooks takes all the historically confusing and time consuming tracking needs of sending estimates and invoices to your clients, and offers a solidified piece of tracking software that's both easy to use and highly powerfull. I don't want to have to worry about emailing invoices, tracking clients and trying to figure out if I got paid or not.  That's why I use Freshbooks, it takes care of that for me.