All you need to do is upload an image (or URL) sample of the font being used and WhatTheFont goes to work providing 3-5 possible options for what the font may be.
Take for example the font 37 Signals have used for the text on their homepage. I’ve always been a fan and that was the first to come to mind when selecting a font to detect.
1. I took a screenshot of that area of the page.
2. Uploaded the shot to WhatTheFont and after confirming the letters of both words, I was presented with the following options:
It has to be said, not bad – not bad at all. Clearly this isn’t a tool you’ll be using on a daily basis but it’s great to know it’s there and it works. Definitely one to bookmark.