WTFPL
The Do What The Fuck You Want To Public License, whose only condition is its name.
Banlu Kemiyatorn wrote the first version around 2000. Sam Hocevar, later Debian Project Leader, released version 2 in December 2004, and that's the one people use. It consists of a line allowing the licence itself to be copied and modified as long as the name changes, plus a single clause: do what you want. No warranty disclaimer, no copyright notice that has to travel with the code.
The FSF classes it as free and GPL-compatible. The OSI never approved it, and the missing disclaimer keeps cautious companies away, since the author has in theory disclaimed nothing at all. It is mostly a statement that licence proliferation is absurd, made by contributing to it.