FontFonter lets you preview a website with FontShop's commercial fonts
FontFonter is a neat tool from FontShop that lets you render a live copy of any Web page using FontShop's proprietary Web fonts. Font prices are in the hundreds of dollars (the "affordable" ones are around the $150 mark), so it's certainly nice to be able to preview what your site will look like with such a font before spending all of that money.
It would be truly great (and magnanimous, even) if FontFonter let you preview fonts from Google's free offering, or if FontShop offered some of the fonts for free (perhaps in certain weights or in limited character sets, as is often done with desktop fonts).
As it stands right now, this is a wonderful tool that is somewhat hampered by business considerations. Still, if you're curious as to what your site would look like with a different font, don't hesitate to take it for a spin. It worked well when I tried it, and it was very easy to use. And if you find a font that you like, you can always look for a similar one from Google's free offering (or just pay up and get the font you like, of course!).