Extensis, which provides a webfont service called WebINK (see New Web Font Services Expand Choices), has launched the coolest tool yet for experimenting with web fonts: the FontDropper.
You don't need a WebINK account to use this; just go to the FontDropper page, scroll down until you see the bookmarklet, and drag it to your browser's bookmarks bar.
Of course, there's no changes made to your source files, since FontDropper has no access to those; all the changes are made in the local copy being displayed in the browser. It's sort of like Firebug for fonts. Try different fonts until you get the look you like, and then set it up in your page's code to make it permanent.
You can get a feel for the possibilities this opens up in just a few minutes. Highly recommended.