TypeTester

The Typetester is an online application for comparison of the fonts for the screen. Its primary role is to make web designer’s life easier.
FontTester

Font Tester is a free online font comparison tool. It allows you to visually compare fonts side by side with different CSS font styles applied. It is very useful for web developers who are looking for just the right font/style/color to use in their pages.
CssTypeSet

CSS Type Set is a hands-on typography tool allowing designers to interactively test and learn how to style their web content.
TypeChart

Typechart lets you flip through, preview and compare web typography while retrieving the CSS. Previews captured in Mac OSX 10.5 and Windows Vista.
Colour Contrast Check

The Colour Contrast Check Tool allows to specify a foreground and a background colour and determine if they provide enough of a contrast "when viewed by someone having color deficits or when viewed on a black and white screen".
http://www.snook.ca/../colour.html
STC FontBrowser

STC Font Browser is a free online flash application that allows you to preview the fonts active on your system.
WhatTheFont

Seen a font in use and want to know what it is? Submit an image to WhatTheFont to find the closest matches in our database. Maximum file size: 2MB.
http://new.myfonts.com/WhatTheFont/
IdentiFont

Find a font by name or identify a font by answering questions about its appearance. Welcome to Identifont, the largest independent directory of typefaces on the Internet. Identify a typeface by answering a series of questions.
Linotype Font Identifier

Answer a few simple questions and the artificial intelligence of the FontIdentifier will help you find your font.