If you have a printed document with text that you like the look of, and you want to find out what font file you need to reproduce the look, this site is for you.
Basically you need a picture of the font, so you'll either need to scan the document or crop a picture depending on your facilities.
The recommended file format is a grayscale TIFF at 300DPI. I've checked a couple of fonts through this site and it's given me excellent results.
Aside from it's practical use, it also offers a great UI when identifying the individual letters that you have scanned. It identifies the 'dot' and the stroke of an i as individual letters, but all you have to do is drag one over the other and the system will merge them together.