Along the same lines is http://hyperpolyglot.org/ (which has been posted here a few times). This is useful both for learning a new language and for a quick reference for languages you're not so hot on. It's also very interesting to compare similarities and how languages have influenced each other.
It seems (from the comments here and my own experience) that many people really like this style of learning. It would be great to see it applied in more areas.
I was so happy with Lua and thought it was such an elegant and clean language.. then I looked at the side-by-side comparison between Lua and Io on the site you linked.. and now it seems clumsy and ugly.
Unfortunately a language is only as good as its implementation and tooling and Io's implementation is "experimental" at best (and development is pretty much dead) + the utter lack of tools and third-party support so I will have to stick with Lua.. and try very hard to forget Io again.
It seems (from the comments here and my own experience) that many people really like this style of learning. It would be great to see it applied in more areas.