In self defense situation a good hard strike to the points of the leg which are 'weak' will give you time to run away. So a stomp on the kneecap and down the shin with a shoe causes discomfort and if you get the kneecap right you can disable them from walking for a while.
If you have a pointy object then you can drop down and direct a blow to the main nerve runnings which I think are in the centre of the thigh (front) halfway between knee and hip, and the inner thigh between where teh main front and main rear muscles meet again about halfway between the knee and the hip (My description here is not medically accurate, but you get the picture). Although it would be rare that you find yourself in a situation where you can do that effectively without years of practise.
Also forcing the knee to extend in adjacent angles to the joint can either disable or cause an upset balance (kick their knee sideways or backwards, sideways is best as there isn't a knee cap to prevent it bending).
In sparring we use many techniques to upset balance, mostly based on rotation and positioning.
One of these is the Naihanchi stance
Link.
Before everyone gets up in arms about horse riding stances let me explain that we don't stand in a fight like we are crapping out of our Gi bottoms, I am mearly about to state one way in which a portion of this stance can help to upset the balance of an opponent (And thats my opinion not a fact, only it works for us when we use it inter-club sparring)
Basically if you move your right knee to point to the right of the opponent then slide your foot diagonally forwards (left a little then right) tucking your knee under their knee (helps if you stand a little pidgeon toed), then return your knee to normal position you will lever their knee and leg in a way that they cannot use it as a supporting structure. Depending on which leg they have in front you will either cause them to lose balance or drawn them into you (note this is not a static technique, usually I use it when the opponent is advancing and I want to influence their direction of travel. So far the people I have sparred moveback too far to use this offensively.
Another one which is a favourite of the flashy points fighters is to perform an axe kick (gedan) into leg sweep (same leg) and then continue turning into a spinning cresent kick (gedan or Jodan depending on how flashy you are).