Font Viewer is a simple utility for comparing fonts. You pick some fonts from your computer, you enter some text, and you can quickly switch between them to compare them. There's no server involved, all it does is add the fonts to the page.
Font Viewer was created to analyze the differences we were about to encounter when switching font formats in my project at work. It was largely developed due to the fact that Windows 8's built in font viewer didn't know what to do with .woff files, and most available projects looked like they'd also install malware on my machine.