This.
I am trying to only use FOSS fonts, also for print, so having families matters even more to me. I actually went through the while directory and sorted it by hand just last month, so this service will prove invaluable next time i need to choose fonts for any project, especially cross-media.
Personally, I would use this tool to find a font that could be used throughout the page for different elements: such as body text, heading text, quotations, etc. The alternative would be to mix and match different fonts, but this takes a lot more effort and experimentation.