If you've ever freelanced, you'll know that having a professional and meticulous administrative system can some (most) times give way to the creative process and getting a kick ass piece of art or writing out by the deadline.
Some clients are insistent about being invoiced and receipted for work done, and its usually a good idea to keep a track of what you're selling and for how much. Doing this yourself usually means drafting some kind of document in excel, or using Microsoft's functional but hardly glamorous invoice templates.
If you're looking for something snazzier and easier to use, try billable.me. A fantastic, free, web app developed by mzansi boy Peter Pistorious and Barbara Cilliers. The hyper-editable web-form, with calculating totals, VAT calculations and PDF exporting gives you simplicity and flexibility in exactly the right amounts. And with no sign up or account required.
Gorgeous, functional and simple, billable.me is a fantastic free to use tool that everyone should use.
Hint: If you're a freelancer and not sure what to charge clients, use the Southern African Freelancers Association suggested rates, which comprehensively covers suggested industry prices for freelance work in photography, design, videography, copyrighting, translation, and writing.
If you're looking to design your own invoices and want some inspiration, Smashing Magazine has a fantastic gallery of designer invoices.