Through The Viewfinder (TTV) is a neat way to use old cameras. A lot of cameras are out there that are neat old relics, with great styling, and they can be had cheaply. Unless you're like me, chances are those cameras are pretty useless in your digital repertoire. Useless as cameras that is, but perhaps not as lenses.
Basically all you need to do is take a camera with a nicely bright and large viewfinder, say an Argus Seventy-Five, and point your lens into the viewfinder. Take a picture, and you have a TTV shot. To be done well, you need to make a somewhat elaborate mask for the finder so that your camera is attached to the vintage camera in a way that makes shooting simpler. Most people use cardboard.
There are lots of folks out there doing this, and as you might expect, I've tried it myself. I find I enjoy actually shooting film through these cameras, but many others have gotten some great shots TTV. Many, like this one, feature strange colors (done in photoshop), and lots of dust (a relic of using these old viewfinders as lenses. I like the effect in some circumstances, though some folks use a photoshop action to simulate the effect. To me that's cheating, because half the fun is knowing you're doing something that requires a bit of effort on the photographer's part, with a creation you've made yourself.
So next time you see an old camera in a junk store, and you're a bit low on creativity, try some TTV photography. It might get you hooked.