www.travelocity.com is the one I use. You need a password, for which you have to give some information, but they don't subsequently bug you with junkmail.
However, when you mention it in your FAQ, add a caveat that you should use travelocity to find your flight, but never to book it; you can always get a better deal elsewhere.
If you add the URL's of all the airlines which allow online booking, this would be a great help, but would be a lot of work for you.
If you could add to this the list of those that take you 90% of the way through the booking process and then crash with a java error
, this would be superlatively useful. In my experience, this includes all of them except one - virgin express's. A shame that their flights are not to the same standard as their website.