Combining Simply Invoices with the tick OSX widget is awesome. I just hit F12 and start my clock, then when I'm finished the data is printed to an invoice. Okay, so there's one step inbetween, but really it's simple. and best of all, it's free! If you want to manage multiple accounts you can a bit pay for an upgrade.
http://www.simplyinvoices.com
http://tickspot.com
