The Internet archive has a fair collection of retro-games you can play inside your browser.
There are only two little problems about that:
1.) The emulation is painfull slow
2.) You can't keep save games

Lately I've done a few investigations how this whole emulation-inside-the-browser-thing is working and it should be possible to write a little script that caches the rom and allow you to play it with your prefered emulator (and delete it afterwards).
Sadly I am not sure if this would be still 'streaming' and legal.