1) Any damn fool can predict the past.
2) Never fire a laser at a mirror.
3) Mother Nature doesn't care if you're having fun.
4) F × S = k. The product of Freedom and Security is a constant. To gain more freedom of thought and/or action, you must give up some security, and vice versa.
5) Psi and/or magical powers, if real, are nearly useless.
6) It is easier to destroy than create.
7) a) Never throw nasty stuff at an armed man.
9) Ethics change with technology.
10) There Ain't No Justice. (often abbreviated to TANJ)
11) Anarchy is the least stable of social structures. It falls apart at a touch.
12) There is a time and place for tact. And there are times when tact is entirely misplaced.
13) The ways of being human are bounded but infinite.
14) The world's dullest subjects, in order, are
(i) Somebody else's diet.
(ii) How to make money for a worthy cause.
(iii) Special Interest Liberation.
15) The only universal message in science fiction: There exist minds that think as well as you do, but differently.
Niven's corollary: The gene-tampered turkey you're talking to isn't necessarily one of them.
16) Never waste calories.
17) There is no cause so right that one cannot find a fool following it.
(also given as "No cause is so noble that it won't attract fuggheads.")
18) No technique works if it isn't used.
19) Not responsible for advice not taken.
20) Old age is not for sissies
21) "There is a technical, literary term for those who mistake the opinions and beliefs of characters in a novel for those of the author. The term is 'idiot'."
The term "Niven's law" is also used for the argument that if time travel and changing the past would otherwise be possible, time travel will tend to eventually eliminate the circumstances in which it actually happens.
* I bowdlerised this law