The Gnaural Java applet allows you to playback binaural beat sequences in a web browser – arrange your own programs using the Gnaural software or simply download example programs and load them via the java interface.
If you’re looking for something a little ‘flashier’ – check out this visual HTML Dreammachine. Far simpler than Gnaural, it allows you to control the rate and color of basic full-window flashes.
You can get acquainted with the mental medium via this handy frequency chart listing each range’s associated states of consciousness.
(Theta [4-7hz] FTW!)