WhatTheFont is a site that lets you upload a logo or any other image that contains text and shows you a list of fonts that are likely to be used in that image. WhatTheFont supports some common image formats like GIF, JPEG, TIFF, BMP, but you can't upload images bigger than 360 x 275 pixels.
After uploading an image, you may need to enter the corresponding letters from the text, since the system can't always detect them accurately. In the example below, the site recognized the font used in Google's logo: Catull.
The site seems to be pretty old, but I couldn't find a similar online font recognition system. Identifont asks you some question about the font's appearance, so it's more difficult to use, while FontExpert only handles individual characters.{ via Daring Fireball }