Simple Invoices delivers, it's not complicated, and does invoicing well. Sending the invoice as PDF with a short note to customers is a real timesaver. The default css template is not very nice but the simplyinvoices.org website had another one which I liked better. If you put your own logo it looks clean & professional. Recommended for people who want THEIR data on THEIR servers, and do not want to pay a monthly fee for a sexier alternative.