[alblue]

It's not an OSI approved license, nor does it actually grant any rights. If you use the WTFPL then you are preventing others from legally using it.

[ronjouch]

Yeah, good to mention it here explicitly. The Wikipedia page linked above has a nice summary of its "compatibility":

  DFSG compatible   Yes
  FSF approved      Yes
  OSI approved      No
  GPL compatible    Yes
  Copyleft          No
  Linking from code with a different license  Yes