BFXR is a free sound synthesizer, specialized to let you make 8-bit/16-bit game sounds on the fly. It no only has very cpable options and a reliable export functionility, it also comes with multiple sound generators – such as “coin”, “jump” or “explosion” – to get you started with one click.

You can use the app in your browser or download the standalone app here: http://www.bfxr.net/