I've been using Duolingo for a few months now in order to improve my French. They've just updated their web service to match their mobile apps and it's a lovely piece of design. My favourite has to be their icons (presented below) – grey for a locked lesson, full colour for unlocked/in-progress and gold once you've completed it.
|Duolingo's lesson icons|
What's even nicer is that they're using SVGs for their icons, meaning they look just as beautiful on retina displays as they do on standard ones.