TypeWonder is a useful tool if you want to quickly see what a live site would look like in a different font. Just enter the URL of the target site, then select the font you want to use from the searchable list that pops up. There's a button that gives you the embed code when you find a font you like.
It's especially useful if you want to preview lots of fonts in quick succession, as you can switch from one to the next with just a couple of clicks. This simple functionality alone is great, but you may feel limited by the fact that only one font can be used at any one time - there's no way to try out font pairs by applying different typefaces to different areas.
If you have need of this service on a regular basis there's a Chrome extension which adds the functionality to your toolbar, with Firefox and Safari extensions to follow.
TypeWonder is in beta at the moment and is free to use.