FreshBooks - Online Invoicing, Accounting & Billing Software if you do a lot of invoicing
http://www.kashoo.com if you need an actual accounting / bookkeeping system. I use it, it's fantastic. Actually I use both. Freshbooks does my invoicing and estimates for clients, and tracks my expenses as well. Everything gets dumped to Kashoo for reconciling bank accounts, doing payroll, taxes, blah blah etc.
Highly recommended