Simple Invoicing Online – free forever
getinvoicing.com
getinvoicing.com
You might want to address that with your marketing / splash page. Maybe a "What's the catch? ..blah blah blah there is none" or "Too good to be true? ...blah blah blah." header.
If you're interested in a really free (as in no registration) invoicing tool, http://invoiceto.me/ works a treat for me. Been using it for months now.