Non copyleft licenses are considered as permissive licenses, mostly because they allow creating derivative works under other license terms.
It is important to make a difference between Free software and Open source software. All Free Software is Open source Software, but not all Open source Software is considered as Free software. For the Free Software Foundation the difference is philosophical, and the Open Source Software initiave would not focused in ethical reasons. The Open source initiative has its own manifesto, and is focused in software development.
It is much easier to classify FOSS software from a legal perspective. We classify it into FOSS licenses with copyleft, and FOSS licenses without copyleft. The Free Software foundation will determine if the license qualifies as Free software, and The Open source initiative will determine if the license qualifies as Open Source. Most FOSS licenses qualify as both.
Some of the most popular non copyleft FOSS licenses are:
– The Apache license. Public license created by the Apache foundation. Current version is the Apache license 2.0. It is a permissive non copyleft license which permits sub-licensing, and reusing it with proprietary applications. Well known software as the Apache HTTP server, the Android OS, or Twitter, are licensed under the Apache license.
– The MIT license. The Massachusetts Institute of technology license is a permissive non copyleft license, allows sub-licensing, and permits creating commercial applications. It is also known as Expat license, or X11 license.
– The BSD license. The Berkley Software Distribution licenses is a non copyleft, permissive license. There are 3 BSD licenses, but only two accepted and recommended by the Open source initiative and the FSF: The Revised BSD license(3 clause), and the Simplified BSD license(2clause). The original BSD license(4clause) is not accepted as an Open Source license, and is not GPL compatible, due to its “advertising clause”. All BSD licenses don’t mention sub-licensing. Well known software such as the FreeBSD OS, or the Metasploit framework, are licensed under the BSD license.
ADVANTAGES: (1) They are permissive, most of them allow sub-licensing, creation and distribution of derivative works without restrictions. (2)Very convenient for developers. (3) No license compatibility issues either with copyleft licenses, or with proprietary licenses.
DISADVANTAGES: (1) Derivative works can become proprietary. This affects final users future freedom(in terms of the FSF).