Xemu emulator demonstrations compiled from Xemu source with the emscripten emulator, so you can try them with only a web broser. WORK-IN-PROGRESS
All of these emulators are available for native versions as well, for Windows, Linux (possible other UNIX'es) and MacOS too, which may give you a much better experience. These are just for quick preview purposes.
NOTE: this requires relative decent browser, almost no hope for IE, and probably even Safari is not decent enough (?), I would recommend up-to-date version of Firefox or Chrome, probably (what we need: HTML5 canvas 2D context, ArrayBuffer, WebGL, asm.js and WebAssembly, File API, WebAudio, Game API support). In theory game controllers should work via your browser too, if your browser supports it (though you can have axis/button-mapping problems).