It's an interesting idea... You could use the "Cloud" concept and store your project files there, but you'd need something that could access them, and yes you'd have to download you executable to run it of course. If it was Java based then possibly you could have it compile on your system, but then again you'd have to download the compiler each time you want to use it instead. Then again maybe you could cache the Java app which might be possible in most web browsers and only have to do it the first time. (or whenever the cache is lost)

There is a bottleneck either way in your idea so it might not be the most catchy idea, but it is a neat one.