Chromoscope shows you the sky in a wide range of wavelengths, from gamma rays to visible to microwaves. It superimposes and registers imagery from various sky surveys and provides a handy slider to change between spectral ranges (the picture here shows an area around Gemini in transition from visible to hydrogen-alpha). You can zoom and pan with the mouse, and there are also keyboard shortcuts (press H for help). It runs directly in your browser, nothing to install (it doesn't seem to use Flash - I tried it on Safari on the iPod Touch, and it loads so it's probably not Flash, but the controls don't really work). Thanks to Idle Diffractions for the tip.
But there's also a free iPhone/iPod Touch app (Planets) that uses the data from this site to show the sky in different wavelength bands. It's called "Planets" but it actually shows the whole night sky, not just planet data.