A file that's both an acceptable HTML page and a JPEG (view source on it)
If you think that's cool, look at Daeken's Magister: <a href="http://demoseen.com/windowpane/magister.png.html" rel="nofollow">http://demoseen.com/windowpane/magister.png.html</a><p>A PNG that's interpreted as HTML and loads itself as compressed JavaScript!