Find Fonts by Sight
At Fonts.com answer questions from a sample of the typeface you are looking for. You will then be shown a list of matching typefaces.

FontExpert
Based on a special algorithm and a huge font database FontExpert is able to identify typefaces from scanned documents; try it online or buy the CD ROM.

Font-Finder
Font-Finder have put together their own collection or resources here in hopes that it will assist people in finding out more about the fonts they're interested in.

FontMatch
FontMatch searches the computer hard drive to electronically match and identify the font you are trying to match visually with the naked eye. FontMatch eliminates having to manually search through all of the fonts installed on your computer one at a time.

Font Matcher
Are you searching for a printed font and you have 9000 TTF's? Font Matcher helps you do that in a few minutes. Just get a bitmap of the letter, select your fonts directory and Font Matcher will compare each TTF with the given bitmap, giving you an exact percent of how much each font matches the given bitmap.

Identifont
Unique font identifier that enables you to identify a font from a sample by answering a series of simple questions.

Name This Font
A Practical Encyclopedia of Letterforms in 2 volumes covers 19,000 letterforms or 740 fonts.

WhatTheFont
Upload a scanned image of a font and WhatTheFont will show you the closest matches in their database.