Work out. Read. Watch movies.
Play video games, but not online because that just stimulates more aggression.
Depends on what was happening and what kind of stress. If I wanted to punch the shit out of someone for wasting time or doing something stupid, then doing something physical is necessary. If it was just a mentally stressful day and I wasn’t actually angry with anyone, then using my brain in a different way relaxes me best.
I go walking. It’s good exercise and I find it soothing.
Run. Or lift heavy things. Or run THEN lift heavy things.
And then kill the shit out of stuff in Rift or WoW, if I’m not in school. If I’m in school tough shit, I study.
Usually, just walking in the door of my house helps a great deal. Kids run over, wife gives me a hug and a kiss, yummy cooking smells fill the place… it totally changes the dynamic of what was going on in my head all day. If necessary, a glass of wine or two with dinner really helps.
And if that’s not enough, after the kids go to bed I let my wife work her magic on me. 
Stressful day? What stressful day?
Play banjo and drink beer.
The usual stuff everyone does: rummage through the shoe box for the spoon, cotton balls, piece of rubber hose, and whichever syringe has the cleanest-looking needle. Tie off a limb and palpate frantically looking for a vein that hasn’t collapsed. Et cetera, et cetera…
I come home to two hungry/tired kiddos. Relaxing activities don’t fit into my schedule after work much. I get the comfy clothes on, hug/kiss the kids and have a diet pepsi, perhaps a quick walk if my hub is home. Sometimes a little pill helps on the really rough days 
I’m with Muddy Waters on this one. Champagne and reefer.
Draw a detailed picture of the worst part of the day and fart on it.
Live nude lesbian sex show at Suzie Wong’s or Sheba’s.
Cram all the anger and sadness down into a little tiny ball just below my belly button. Just keep on cramming it down, pushing all those feelings deep inside, until they all go away.
When do they go away?
[Runs sobbing into the other room.]
Normal day - A couple beers.
Stressful day - All of the beers.
Drink heavily at home, or go to a favorite microbrewery to have some conversation. Usually can’t concentrate well enough to read what I’m working on. Sorry, Ingarden, I need my full wits for thinking about your controversy over the existence of the world. Same with you, Mallarme’, you smarmy little French bastard. Sometimes I just pop a doxylamine sleeping pill and a Xanax and do some crosswords until I get to sleep. I wouldn’t rule out masturbating like a motherfuck, but that takes some effort. Or, more often, I watch some crazy movie from the 1970s I’ve seen a million trillion times and zone out, and skip the crosswords until I’m in bed. Also, playing some piano like a motherfucker works too.
Change my clothes. Hanging up my suit and closing the wardrobe door make it definite that I don’t have to go back to work. Have a cup of tea. Maybe do a bit of gardening, maybe do something to the car (I find using the foot pump to top up my tyres helps on the days when I want to kill someone), maybe play the guitar. Cook. Check my email, read the Dope with the TV in the background. I used to look at facebook, but since they put this stupid ticker on it just annoys me and doesn’t tell me anything useful any more, so there’s no point.
I agree that the changing of clothes is a much-underestimated step in the chilling process.
Get on my mountain bike and hit the trails behind my place after work for an hour.
Put on radiohead tune out and paint.
Bass.
Guitar.
If all elese fails and it turned out to be a really bad day copious amounts of alcohol. Only had to do that once.
Uh. What a boring thread – why not more answers? OK, fine, I eat some bitch out, then jerk off into her snapper, then eat a pizza while changing my clothes, and then pick up my leather-bound Pleiade edition of Vale’ry (vol. 1) and listen to Bach while smoking my pipe.
Jeez, talk about a boring-ass thread.
LMFAO!!! Wha?!?
Well, I guess a man does have to change his clothes after a stressful day…
Well, maybe you and I have different techniques of eating supper.
Work stress is part of why I started learning to cook. My ex made some noises once about how I shouldn’t have to do all the cooking after working all day. I told her it was better to chop up vegetables than to chop up her and the kids.
But, kinda, isn’t that the point? Sure, it would be great to bark out some macho crap, but ultimately, you need to find a way to check out that works for you.
In your case, you play, based on how you are able to describe it, some kickass keyboards. If I could boogie like Pinetop Perkins or Booker T., clearly that would be an advantage when looking to blow off steam after a stressful day.
As for myself, I just got a new guitar toy (a stompbox delay), and I spent a solid hour this afternoon rocking out on guitar. How do I blow off steam? That’s pretty much it - with some reading a yoga tossed in…