Blinksale is not the only utterly simple invoice generator out there. There's another utility, Simplybill, that offers a lot of the same functions, such as very easy data entry, a clean view of outstanding and overdue invoices, and a simple system for creating recurring bills. Paid customers of Simplybill also get access to a downloadable time-tracker widget, which could be very useful for consultants.
Simplicity is a good thing when you have a simple job to do, but I fear that Simplybill has gone overboard. For example, when you want to e-mail an invoice to a client, one press of the "e-mail" button sends it. That's simple, but you should have the opportunity to at least add a cover message or attach CCs to the address. Worse, there's no way to edit an invoice that's in the system, which is a bummer if you've entered a bunch of data into an invoice but realize after that fact that you haven't changed the default 10 percent sales tax.
Like Blinksale, Simplybill is free for up to three invoices per month. For the unlimited plan, Simplybill is cheaper: $25 per month, vs. Blinksale at $49. Both products have competitive intermediate pricing plans.
For the moment, I recommend Blinksale for simple invoices.