I have recently discovered that occasionally bursting into evil laughter is actually quite calming. However, it's not going to work if you just laugh. You have to let it come to you.
Shut your eyes, and imagine:

You are a evil villain. The archetypal bad guy characterized by Bond and countless others. Your quarry lies at your feet, the once sleek and suave agent sent to depose you now a broken shadow of their former self. Their bow tie is undone, their rolex is broken. They are beaten. You have won. You, the mistreated genius gone rouge/disillusioned agent gone rouge/badger gone rouge are now free to rule the world with an iron fist. You death ray is primed, you henchmen stand ready, and your plan of evil is ready for the world.

Now let the feeling you have bubble up inside of you, each individual dome a 'Ha'. Let the bubbles move up inside you, coming up your throat. Let a small few bubbles escape, again and again, becoming more numerous and powerful. Build up your laugh, moving up registers until you are at crazy laugh level and beyond.
Stop if you can.

Oh yes. Yes it most certainly is.

oh yeah it defiantly is.

I have several friends who would agree with me on that too......guess that says something about the people I hang out with.

Sonicron:
I've discovered this long ago. Incidentally, I find that maniacal laughter (i.e. Joker style) is even more liberating than evil laughter.

I agree. Crazed, deranged laughter is just more effective, probably because you don't have to think about it at all. If you're really feeling stressed, you just let the hysteria out.

I agree. I don't need all of those descriptions to really do it, but it's not just an evil laugh. For me, it's more of a Magnificent Bastard laugh. Which may or may not be evil. In my case, not.

...Except in Half Life 2: Deathmatch. I am absolutely insane in that game. If I laugh there, it's because I just got my 15th straight rocket launcher kill. Which would be a diabolical laugh indeed.

I love diabolical laughter, especially at work. It really freaks 'em out there.

I usually find the gun range to be extremely therapeutic. As is laughing like Dr. Evil.

I'm told my evil giggle is for some reason more intimidating than my evil laugh... Although to be fair it's an evil little girl giggle.

Consistent with the relief theory, any laughter is therapeutic.

I usually find myself laughing at the same time as villains but that's usually most definitely laughing at them.

tellmeimaninja:
I always laugh maniacally when I'm stressed. Makes my job a lot more interesting.

Please tell me you work at NASA mission control. That would be epic. The implications of your manic laughter could mean anything.

Even babies know this one

But - there's a reason, I do it all because I'm evil, and I do it all for free, your tears are all the pay I'll ever need.

Laughter's alright but I've found that an evil monologue works even better.
Coming up with all the different ways of saying "I won, you lost" requires a fair bit of mental effort, making you forget whatever was troubling you.

Well, yes, although it doesn't have to be an evil laughter.
It can simply be crazy/maniacal and have the same effect, at least from my experience.