(Simplified, so probably don't base any major life decisions on this list. At least Google it first.)
  1. Rule against perpetuities
    You can't write your will to make things happen on conditions that may be met more than 21 years after the death of a person who's alive when you die. I like this because of the concern about control by the 'dead hand' - envision a society where I suddenly get all your shit because you violate a condition in a will drafted 200 years ago. Fun!
  2. Adverse possession
    If you just take over and start living in someone's house, you get it. This is cool because your use has to be 'open and notorious,' which is badass. Also you have to be using the property without the owner's permission, and I think it's funny that you HAVE to be terrible to win. Yay law!
  3. Eggshell skull plaintiff
    You are responsible for the results of your negligence, even if the victim was unusually vulnerable and you couldn't have known the injury would be so bad. This is cool because of the vivid imagery -- imagine rapping lightly on someone's skull and it just caves in like an egg, eek.
  4. Res ipsa loquitur
    'The thing speaks for itself.' There's a (rebuttable) presumption of negligence, where something doesn't generally happen without negligence. The classic example is a scalpel left in a patient's body after surgery -- the surgeon was probably negligent. This phrase is VERY useful for fighting with loved ones in everyday life; the Latin throws them off.
  5. Impossibility (Factual vs. Legal) in Criminal Law
    Suggested by @bobbyhundreds