Web application Flipping Typical detects and previews installed fonts on your system with any text you want.
The application aims to determine every font you have installed on your computer and preview them all, but in my testing I found that it left out some of my fonts. Whether or not it automatically detects all the fonts on your system correctly, it's still an impressive and fairly useful app for comparing fonts available on your computer without downloading anything. And if you know a font's installed on your computer and you want to see it included, you can type it in as the top preview font and voilà—you've got your preview.
Flipping Typical is a decent site and perfect for a quick font comparison, but if you're really serious about fonts, check out our favourite typography tools and free font downloads.