The site doesn't quite live up to the title — you're not actually going to completely learn a language using it — but it's very neat nonetheless. I find that it

The site doesn't quite live up to the title — you're not actually going to completely learn a language using it — but it's very neat nonetheless. I find that it works well to get a quick feeling for a language that I'm considering to pick up, or as a refresher on the syntax of a language I haven't used in a bit.