One thing I love about OS X is that no matter how long I use it, I'm often surprised by something that I didn't expect to find. That was the situation last week when i was using Apple's Preview app in OS X 10.8 Mountain Lion to resize an image, and found myself looking at a part of the image through a magnifying loupe.
The loupe moves wherever your cursor is, and provides a nice magnified view of whatever you happen to be pointing at. While images might appear a bit pixelated within the confines of the loupe, it's a perfect way to get a closeup view of details that you might otherwise miss.
I accidentally made the loupe appear when I was trying to press the "1" key on my keyboard and pressed the "grave accent / tilde" key instead. It's the key just above the tab key and just to the left of the "1" key on the US English keyboard. The loupe toggles on and off with each successive press of the key.
Sure, there's not much earthshaking about this feature, but it is both useful and fun to show off to friends who think that there's nothing new to find in OS X.