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.

a bookmarklet for typographically obsessed web designers. It enables rapid checking of fonts and font styles directly in the browser without editing code and refreshing pages. 2.0’s killer feature is instant drag-and-drop font previewing right in the browser (Firefox 3.6+ only), in any document you’re currently viewing.

... is an online application for comparison of the fonts for the screen. Its primary role is to make web designer’s life easier. As the new fonts are bundled into operating systems, the list of the common fonts will be updated.