As a small business owner / freelancer one of the most annoying things for me is chasing the money. I love having the money in my account, and I love doing the work to earn it... but the bit in-between is time-consuming and sometimes very frustrating. Over the years I've tried a number of solutions and never found the right one.
FreshBooks seems to be a pretty good solution though. And it's a very low risk trial (free if you only have 3 clients or less).
Account management, payment integration and setup is very simple - took less time to catch up on outstanding invoices for about 6 jobs tonight that I usually take with basic account management each month!
For clients where it's a simple case of billing against a PO or fixed price quote the process is really simple. For clients where you need to be able to produce a timesheet to track billable hours that's also possible (though only via web based time-entry at the moment... can only hope a desktop/PDA add-on eventuates at some point - for Windows, OSX, Windows Mobile, Palm and Symbian...a tough call I guess, but no harm in hoping)
The system automates a lot of stuff that you either have to set reminders for (or pay your accountant extra to do) in other solutions and has seamless integration with a number of payment gateways (from a very simple PayPal solution, up to high end credit card processing solutions).
What makes FreshBooks so interesting is that it's not a big accounting firm, but a small 6 man team who really seem to love what they're doing (and more importantly remember what folks actually need the system to do)