Long lenses are a popular choice for portrait photographers but if you're short on space or you want to create an unusual portrait pick up a wide angle lens.
Wide angle lenses are great if you want to include a large amount of background while still keeping your subject full in the frame. They're also great if you're working in a tight space and you want to shoot a full length portrait. Due to them exaggerating size they will also make a person appear higher off the ground than they are if you shoot up at your subject.
If you use a moderately wide angle lens you can still shoot portraits that don't distort the subject but do increase the size of objects in the foreground of the image. They're also good for exaggerating movement, such as someone swinging towards the lens.
You can also use them for shooting candids while out on the street as the wider angle will mean you can still capture your subject in frame without actually pointing the lens directly at them.
These are rather sensible shots but if you do want to have more fun then the distortion of a wide angle lens will let you do exactly that.
If you position yourself so you're looking up at your subject and shoot a shoulder/head shot your subject's forehead and chin area will be exaggerated while shooting a full length portrait will increase the size of their feet and lengthen the legs. Look down on them and you'll give them cartoon-like eyes and a giant nose while the rest of the body will shrink in comparison. Just remember that the closer the subject is to you the more obvious the distortion will be. Go even wider with a fish eye lens and you have the opportunity to produce a very comical shot.
The above shot was taken on an iPhone with a USB Fever 190 degree iPhone Fisheye lens. If you use an iPhone for quite a bit of photography consider buying a KLYP iPhone case that makes it possible for you to attach accessories such as a pocket tripod or LED Light to your iPhone.
If you position your subject in the centre of the image, due to the barrel distortion they will be magnified while the edges of the image will be curved, drawing the viewer's eye to the centre of the frame.
They're not the lens for someone wanting to capture a true likeness of someone but they are great for a few hours of fun.