[sfricks]

Stretching the leg muscles improves muscle flexibility and strength, running speed, and jumping distance, according to a study from Louisiana State University (Medicine and Science in Sports and Exercise, October 2007). Stretching elongates muscles and tendons. Longer tendons allow muscles to exert a greater torque on the joint to exert more power to help you lift heavier, jump higher and run faster.
However, other studies show that you should not stretch before a competition involving speed and strength (Journal of Bodywork and Movement Therapies, April 2006). The longer the athletes stretched, the weaker they became. Prolonged stretching fatigues muscle fibers so that they contract with reduced force.
Do slow deliberate stretches lasting a few seconds to several seconds, rather than rapid hard pulls on your muscles that can tear them. Stretching cold muscles can also tear them and increase risk for injury. Warm up before stretching or stretch after your workout, but realize that prolonged stretching before a competition can harm performance.

[pyschosyd]

Before excersise you should perform Dynamic stretches only. Ideally range of motion. Trunk rotations, arm swings and the good ol' ABC's.

After excersise you can do static stretching of all of your muscles. Ham's, quads, calves, and upperbody as well.

There are a few schools of thought on this matter. Some still suggest that static stretching before any excersise is OK, but it is becoming less common. You can micro-tear your muscles when doing this potentially causing damage and injury.

[Spike10]

Take it from a old martial artist, light stretch before will allow the muscles to get going to full exertion faster without tearing or pain, and allways do a stretch after workouts or racing, this will also make the recovery phase less painfull and quicker.
Spike.