FreshBooks is the most useful web-based tool that I use as a freelancer. The product is easy to use, has great features and they have good customer service.
One pleasant surprise was that they give you a sample snail-mail invoice, even with their free plan. It may be a clever marketing ploy but it works - I remember opening that envelope and being convinced that this would add a whole new level of professionalism to my freelancing.
Yes, I got that, too. :) They also gave me an introductory phone call to see how things were going and if I needed any help. They are really top-notch. :)