Nice and functional - you can change all the standard font CSS properties using the user interface on the left. It's also a quick way to check out different browsers - just load fonttester in each browser and compare the fonts.
For more information on fonts and typography, take a look at the following web devil posts:
- using fonts in web design - display fonts Vs text fonts
- how to choose perfect heading size and text size
- why use ems for font size in web design?