I searched and searched and found nothing on Typophile about this yet, so please pardon any redundancy if this has been discussed already...
I came across FontJazz tonight (thanks @smashingmag via Twitter). Looks pretty good at first sight. Biggest advantages are its small size and that it runs client-side rather than server-side. Not sure about the licensing issue, and the biggest drawback I can see is that text is not actively selectable. What I mean is that you can't select letter-by-letter (as we tend to do), but if you select and copy text that includes the rendered heading, the heading will paste from the clipboard (there's just no visual feedback that you selected the header).
Anyway, they say it's only in beta and promises to deliver more soon. I wonder if that includes info on issues with fonts' EULAs. I'm looking forward to trying it, just not now — it's 2:30 and I'm so going to bed...