Freelensing. What is it? Should I do it? Is it safe (not entirely, so read through before trying)? Will it make me insanely rich? Answers to those questions and more below.
Freelensing is exactly what it sounds like. It's like your camera going commando, but potentially more dangerous. Most modern SLRs will allow you, at least in full manual mode, to fire and capture an image even if no lens is attached to the body. Why? Well, there are plenty of amazing pieces of old glass out there that don't have any of the fancy circuitry to tell the camera what lens is attached, or allow for autofocus or automatic aperture (yes, there was a time not so long ago when you had to adjust your aperture by twisting a dial on the barrel of the lens - crazy!). So, in order to allow you to use glass that the camera can't identify, camera makers have made it possible to trigger your shutter with nothing attached to the camera at all.
So? Well, I'll tell ya.
Now you can take literally any lens, hold it manually in front of your camera's mount, and experiment til the cows come home. Because the distance between the rear element and focal plane is now variable (try moving the lens close and farther away from the camera), and because you're unlikely to get the lens perfectly parallel to the plane, you can now shoot with a crazy amount of selective focus. By that I mean you can shoot such that only a diagonal strip of the photo is in focus, while everything else goes wildly soft. Think tilt-shift, but without the very expensive tilt-shift lens. Now, if you imagine you're going to have any sort of fine control, you're dreaming, but practice and you might start getting some predicable results.
Then, go and take it one step further - flip that lens about face and now you are armed with the world's most ghetto macro setup. 50mm lenses work great for this, as does anything wider. The first thing you will notice when doing this is that focusing is a trifle difficult. Any tiny change in distance either between subject and lens or lens and camera will change the focus dramatically, and your depth of field is about as close to zero as you can get. But that's what makes it fun. Take lots of shots and play around.
Look. I freelensed my bonsai spruce tree!
So, before you go grab your gear and play, consider a couple things first. Any time you expose your lens or your camera body to the elements, you run a not insignificant risk of causing damage, either through scratches or from dust. So, just be aware and very careful of your environment when you do this. And if it's a camera or a lens you care deeply about, try the technique sparingly, as you will (at minimum) get dust on your sensor. Dust can be cleaned, but it's a pain in the butt and takes time (and moolah).
One more thing - most modern lenses do not come with aperture dials, and as such are at rest with a closed aperture. This is no good for this technique, as you want lots of light (especially for the macro stuff). So, either go down to the junk store and buy a cheap-o old lens (recommended), or locate the lever on the back of your lens that operates the aperture and hold it open manually while you compose and shoot. AGAIN, do that with the utmost care.
That's it! If you have an older second body and some crap lenses from Goodwill, then knock yourself out!
Oh, and no, it will not make you rich.