The Unlicense is a template for disclaiming copyright monopoly interest in software you've written; in other words, it is a template for dedicating your software to the public domain. It combines a copyright waiver patterned after the very successful public domain SQLite project with the no-warranty statement from the widely-used MIT/X11 license.
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Pro Removes any possible licensing issues
Everything under the Unlicense is part of the public domain, which means that anything can be done with it.
Pro Protects the author
The author of the software avoids any risk for how the software may be used by others.
Pro Unlicensed code is easy to find
When using public code hosts such as GitHub and BitBucket it is suggested to use the filename UNLICENSE instead of the more traditional LICENSE or COPYRIGHT to store the license so it's easier to find unlicensed code. Additionally, the first line of the license is worded in a unique way specifically to allow searching for it with search engines.
Pro Short and simple
The Unlicense is just 3 paragraphs long.
Con Hasn't been reviewed by OSI
The OSI (Open Source Initiative) is an organization that promotes open source software and they commonly review open source licenses. However, they have decided not to review Unlicensed because it has not been written by legal professionals.
Con Gives no control of how the code is used
Putting the code in the public domain leaves it open to abuse.