If you're the type of person who looks at the design of a webpage, you'll want to download and install WhatFont. WhatFont is a straightforward tool, designed by college student Chengyin Liu, that apparently parses the CSS of a website and pulls out the font used in a section of text. It also tries to figure out the font in an image, but the tool warns you that it's not accurate. From what I've seen, WhatFont uses the CSS font tags of the surrounding text to guess the text on the image. The tool probably assumes websites will keep the font uniform across a page and not mix Helvetica text with Lucida Grande in an image.
WhatFont is one of those basic tools that's nice to have around when you need it. Sure you could do it be hand by looking at the source code, but WhatFont takes the tedious part of parsing code and does it for you. It's available for free from Chengyin Liu's website.