Thought I'd add some more esoteric types of stretching for the sake of completeness:

Forced relaxation - This is isometric stretching in which the tension is held for as long as possible, until the muscles collapse in exhaustion and the stretch can be increased.
Fascial stretching/ Loaded passive stretching - This type of stretching is usually performed between sets of strength exercises in order to increase strength. It involves bearing a weight with a stretched muscle, so that the muscle is being passively stretched out by an external force, while it contracts. The purpose of this type of stretching is to stretch the fascia, the outer coating of the muscle, to allow for an increase in the mobility and size of the muscle.
Digital fascial planing - This type of stretching can be quite painful. It involves rubbing your knuckles (or something similar) along the length of a muscle which is under a fascial stretch. This helps to expand the fascia.
Active isolated stretching - This involves holding a passive static stretch for two seconds, relaxing for two seconds and repeating 2-15 times.
Shutdown threshold isometrics - When any muscle is forced to contract maximally for an extended period of time, the nervous system shuts it down in order to prevent a tendon rupture. This is known as the Golgi reflex. Shutdown threshold isometrics forces stretched muscles to contract very hard (much harder than in standard isometric stretching), usually by putting a heavy load on them. After a certain period of time - anything between thirty seconds and several minutes - the muscle will relax completely, allowing the stretch to be increased.
Plyometric stretching - This lies on the border between ballistic and dynamic stretching and, as such, can be tricky to perform safely. It involves swinging a limb at speed and pulling it back the moment it reaches the limit of its range of motion and the stretch reflex kicks in.
Microstretching - This is static passive stretching done very gently in a relaxed position (often seated or lying down) for 60 seconds. The idea is to allow for full relaxation of the muscle and allow for stretching of the muscle-tendon unit as a whole without the danger of damaging the tendon.

Chen Zen, your information was very good, but I have some small clarifications to make:
The main one is that in your explanation of isometric stretching, you forgot to mention that as you relax after tensing, you should immediately increase the amplitude of the stretch.
Your recommendations about time under tension and repititions for isometric stretching are fine in themselves, but a little too prescriptivist. The number of reps should be as many as you can while still being able to increase the amplitude upon relaxation. In my experience, tensions longer than 3-5 seconds give no advantages to flexibility, but will build more strength.
Lastly, your description of PNF sounds to me no different to isometric stretching, and indeed the two are most often used synonymously.