Fix perspective (aka "keystoning") in Photoshop

You know when you take a photo of a building and the perspective makes it look like it's getting more narrow at the top? That's called "the keystone effect" and it can be very annoying. But there's a quick way to fix it in Photoshop, so your snapshots look more professional.

First duplicate your layer (Mac: Command–J; PC: Ctrl–J) so you can toggle between the original and the adjusted versions later. Now, with your top layer active...
In CS5, just go to Filter-->Lens Correction

In CS4 and earlier, go to Filter-->Distort-->Lens Correction

 In CS5, you will have to click the "custom" tab instead of the "auto correction" that appears when the window opens (the "auto correction" doesn't appear in earlier versions, sadly, because it's really cool and I'll deal with it in another tutorial)
A pop-up window will now appear with your image at the left and some distort options at the right. The pictures are pretty self-explanatory. Go to the vertical and horizontal perspective sliders and drag to the left or to the right (go ahead, be extreme at first so you can see what it's doing—or you slide them like crazy and pretend you're in "Inception") until you fix the perspective.

For my photo, I brought the vertical perspective -23 to the left until the towers of Notre Dame didn't look like they were being sucked into space by aliens, and then I shifted the horizontal perspective just a little to straighten the building more. Just eyeball it.

One more difference between CS5 and earlier versions. As you shift the perspective, CS5 crops as needed. In earlier versions, once you commit your transform, you will have to re-crop your image. Toggle the background layer off and you will see what I mean. If your initial image it too tightly cropped then you may end up losing parts of the picture you wanted to keep.

There you have it. A super quick fix for a very common problem.