Ask HN: How do you invoice - especially you freelancers out there?

Hi guys, I'm a relatively young bloke who does some freelance web development around my university and part-time job. This mainly means me either working from home, or driving out to an office or house with my laptop.<p>I'm interested in a simple invoicing solution which generates a professional PDF. I don't mind if the solution is a downloadable program or webapp, however something which I can have running real-time that takes away the burden of monitoring my time would be great, especially if it's integrated and not in itself a separate application. Something that goes further and monitors time spent on a single task would be great too, both for me so I can evaluate time management, and for the customer so they have a much more detailed invoice and are better able to see where their money is going.<p>So, how do you guys invoice, and (if you have time to detail further) what would you suggest for my circumstances?