Have you ever measured the exact resolution of your screen?
By "resolution" I mean the PPI (pixels per inch) value. Confusingly, there are two definitions of the word "resolution" - one refers to PPI, and the other refers to pixel dimensions, which you probably already know - my screen is 1280x1024.
It's really easy to measure your screen's PPI:
1. Make a new blank document in Photoshop, and set it to 1000 pixels wide. (It doesn't have to be 1000 - if you have a smaller screen, maybe 700 or 800 will do.)
2. When the blank document opens, make sure you view it at 100% (by pressing Ctrl + if necessary). You might need to hit your Tab key to temporarily hide all your palettes.
3. Measure the width of the document by holding a ruler or measuring tape up to your screen. (Be careful not to scratch it!) Jot down the measurement in inches.
4. Then just grab your calculator and divide 1000 (or whatever you used) by the width in inches. The resulting value is your screen's resolution.
I made a 1000px image on my screen, and it measured 11.575 inches wide. 1000/11.575 = 86.4ppi.
I bet you ten bucks that your LCD screen has a higher resolution than 72ppi. 72 is an out-of-date standard, but a lot of people cling to it stubbornly.
Ok, so what's the point?
There's only one purpose for this that I can think of, but it's a handy one. Have you ever gone to View > Print Size and been disappointed that Photoshop didn't actually show you the image at the right size?
If you go to Edit > Preferences > Units and Rulers in Photoshop, you can enter your precise value in the Screen Resolution field.
Then, when you've got an image open and you choose Print Size from the View menu, it will give you an accurate representation of your image size on screen.
Plus, if you're as nerdy as me, it gives a warm glow of satisfaction to be able to say "I know the exact resolution of my screen - do you?"