The 
Seizure Inducer
Be forewarned: this effect is intense. At the very least it will probably make you uncomfortable after a while. I find it gives me a slight headache.
Worst case, it probably can cause an epileptic seizure (especially if you're viewing it on a CRT), so don't click the "Start" link if you're not sure you can take it. You have been forewarned. I accept no responsibility for what happens to you from this point on. Proceed at your own risk. But take solace in the fact that it's only going to flicker 1000 times, so it'll stop in about 75 seconds.
If you start to freak, click the "Stop" link and it will reload the page (which is a fairly clumsy but failsafe way to stop the madness).
Needless to say, this page is not endorsed (or, I hope, known of at all) by Nintendo or any other companies that own licenses to Pokémon intellectual property. This page is for amusement (or distress, or amusement at others' distress) only.
For a little added fun, stare directly into Pikachu's eye while it's running, then click the "Stop" link without breaking eye contact. (It helps to place your mouse over the "Stop" button before you stare, so you can click it without looking.) You'll see a drop shadow around Pikachu that isn't really there! There's a white glow around him while the flickering takes place, which will leave an impression on your eyes and invert as a shadow once the white background returns. It's the same principle as the American flag optical illusion where you stare at a reversed-color American flag for a minute, then look at a blank white space and "magically" see the good ol' red-white-and-blue there. (Of course, none of this will work at all if you're using Internet Explorer 6 because of its lack of support for transparent PNG images.)