The result of three and a half days of porting (plus a couple weeks of adding features), RetroArch is now on your web browser! "How useless!" you say? Well probably, but it was a fun thing to do.
Most of the general features of RetroArch are here with some exceptions. V-sync is based off of your browser, so it's not perfect. Sound can also a little staticy. Try increasing the latency a bit to improve it. V-sync can be glitchy because it's not ran on the monitor's actual v-sync but the browser draw event. (Firefox team is working on syncing draws to V-sync, which should help this a ton.) In the meantime, you should keep v-sync off. Also, file saving is not in yet since I don't want to write a bunch of Javascript right now. This also means savestates and stuff don't get saved when you leave the page.
Firefox 22 or later gets the best results with asm.js, especially Nightly versions. You can also get better performance on Windows if you enable native GL (in about:config, set "webgl.prefer-native-gl" to true) but this might not work on all drivers *coughamdcough*. Google Chrome is not as fast, and I struggle to get fullspeed on an i7 4770. I've heard that nightly Chrome has made some big improvements in compiled Javascript execution, so that might be worth trying. Other browsers may or may not work, but don't bet on them.
Coming soon: Mupen64 port with Javascript Dynrec? (don't count on it)
UPDATE: Audio is in now. It's not perfect but it sounds pretty good.
UPDATE2: Audio works a bit better, set v-sync off by default (not sure how we can do this the "right" way in javascript)