Almost any modern web designer now has to decide between potentially thousands of custom web fonts. To rapidly prototype different combinations in the browser, even with good front end development chops, can be pretty time consuming.
Typecast is a web app that tries to address this problem. You get a much more visual, WYSIWYG interface for trying out font combinations, but with the advantages of actual font rendering in the browser. I don’t see a personal need for the app given the cost ($29 a month isn’t a minor expense) and considering at my day job we’re mostly focused on Proxima Nova. But especially in an agency environment, this could be a huge time saver.