There are lots of online invoicing tools. Really depends what all you want as they each seem to have strengths and weaknesses.
For consulting type services
If you are primarily doing product invoices and you don't really care if your customers pay online or get automatically reminded that they owe you money you can use.
Quickbooks has much better reporting, expense tracking and is a full accounting package
My favorite is Cashboard because I primarily do consulting type services, but it works good for product type billing. Customers can pay online and get reminded when they owe you money. You can also configure it to integrate with a 3rd party system so when you 'print' an invoice that other company prints, stuffs and mails it... not you. Freshbooks also has that feature.