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 }