Google Font Directory
The Google Font Directory lets you browse all the fonts available via the Google Font API. All fonts in the directory are available for use on your website under an open source license and are served by Google servers.
Font Tester
... a free online font comparison tool. It allows you to easily preview and compare different fonts side by side with various CSS font styles applied to them.
Typetester
... is an online application for comparison of the fonts for the screen. Its primary role is to make web designer鈥檚 life easier. As the new fonts are bundled into operating systems, the list of the common fonts will be updated.
Typechart
... let鈥檚 you flip through, preview and compare web typography while retrieving the CSS
The Elements of Typographic Style Applied to the Web - a practical guide to web typography