If you need to create an invoice fast, Billable wants to help you out. All you need to do is visit the Billable site, edit the invoice template, and print.
Billable is ridiculously simple. When you visit the Billable website you'll be able to click in any field and start typing. Once you're done entering all your info, you can print your invoice or save it as a PDF. Pretty simple, and definitely useful if you ever need to create a quick invoice. (Note that Australian invoices will need your ABN and the words 'Tax Invoice' added if you charge GST.)