Some of you may know WebInk, the web fonts service from Extensis, the guys behind Suitcase Font Manager. They offer a pretty cool widget called FontDropper 1000, used for live preview of the fonts on any web page, really any web page, no matter if you have administrative access to it or not.
Using it is very simple, all you do is just drag the bookmarklet to the bookmarks bar, and then drag the fonts out of the preview panel on the text: look how I redesigned the sidebar. Of course the fonts selection is somehow limited to the WebInk library, but the most useful feature (for typophiles) is that the tool also tells you what webfont is being used at the moment (the little rectangle with Benton RE is the font used at ATypI site). It definitely recognizes fonts that are not in the library as well (tested on a site with custom font). Very handy, not having to look into the CSS and all.