Source: http://feeds.gawker.com/~r/lifehacker/full/~3/356367996/invoice-for-free-with-curdbee
Free online invoice manager CurdBee generates and tracks email invoices for your client that include Google Checkout and PayPal integration to accept payments. Customize your invoices with a logo and color scheme and bill in various currencies to make life easier on your international clients. CurdBee is free to use. Thanks Sean!