If you've ever tried taking pictures from strange angles, or close to the ground, with an SLR, then you'll know that is a surefire way to either get a crick in your neck or end up with a badly composed photo. Just when you've spotted a great photo you find it required putting the camera in a place where you can't actually get your eye to the viewfinder.
The old fashioned way to get around this (i.e. up until about 18 months ago) was to use an angle finder. These attached to the eyepiece of the viewfinder and gave to a bit more breathing room. But they were expensive and not always very easy to use.
But angle finders can now be relegated to the dustbin of history, as the latest generation of DSLRs all have Live View. In this mode the mirror flips up and you get a live image of what the sensor is seeing on the rear LCD.
With the 5D mark II that I typically use, the screen is bright enough and has a good enough viewing angle that this has become a seriously useful feature, even for non-tripod based work. Stick the camera where you want it, compose on the rear screen, and take the picture.
There are a couple of things to bear in mind though. The first is that autofocus works rather differently in this mode. No near instant AF. Instead you either have to use painfully slow contrast detection AF that will take a few seconds to focus. Or the mirror will flip up, achieve focus with the ragular phase-detection AF sensors, before the mirror flips down again and live view resumes. So if you're hoping to catch the decisive moment this way, then be sure to have focused in advance.
But even with this limitation it allows for some cool perspectives. Both the photos in this post were taken this way on my recent assignment to Fes.
All I wish for now is an articulated screen to really open up the angles.