Software in the public domain disclaims all copyright interest, being freely available to everyone for any purpose, with no attribution or acknowledgement required. There are several ways (summarized below) to dedicate software to the public domain—although be warned that some jurisdictions do not recognize the legal concept of "public domain" at all.
See the Wikipedia article on public domain software for more information.
The Unlicense is a template for dedicating your software to the public domain.
The Unlicense license text can be found at:
Creative Commons Zero 1.0 Universal
CC0 enables scientists, educators, artists and other creators and owners of copyright- or database-protected content to waive those interests.
The CC0 license text can be found at: