While Ruby itself is licensed under a variant of the BSD license, the community around Ruby has generally adopted the MIT license for their open source projects.

The MIT license is a permissive open source license that allows anyone to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the software, as long as they include the original copyright notice and permission notice in all copies of the software.

See how you could actually read that paragraph and understand it, and how it essentially provides a "do whatever you feel like you wanna do" seal of approval for anyone using the software?

Sounds like a pretty good deal to me, and it's one of the reasons why I love Ruby and its community so much. The MIT license is simple, permissive, and encourages sharing, which is exactly the kind of environment I want to be a part of as a developer.

History

It's hard to pinpoint exactly when and why the MIT license became the de facto standard for Ruby projects. The oldest seminal Ruby project I could find that started with the MIT license is rake, which was originally announced in 2003. Probably the most influential project to adopt the MIT license early on was Rails in 2004.