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.
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.
Con It's not global
It doesn't make sense outside of a commonwealth ecosystem, is explicitly illegal in some places (Germany), and of unclear legality in others (Australia). See the discussion on the OSI mailing list: https://lists.opensource.org/pipermail/license-review/2012-January/001386.html.