Why you shouldn't use the Lesser GPL for your next library
See “How to choose a license for your own work” for general recommendations about choosing a license for your work.
The GNU Project has two principal licenses to use for libraries. One is the GNU Lesser GPL; the other is the ordinary GNU GPL. The choice of license makes a big difference: using the Lesser GPL permits use of the library in proprietary programs; using the ordinary GPL for a library makes it available only for free programs.
Which license is best for a given library is a matter of strategy, and it depends on the details of the situation. At present, most GNU libraries are covered by the Lesser GPL, and that means we are using only one of these two strategies, neglecting the other. So we are now seeking more libraries to release under the ordinary GPL.
Proprietary software developers have the advantage of money; free software developers need to make advantages for each other. Using the ordinary GPL for a library gives free software developers an advantage over proprietary developers: a library that they can use, while proprietary developers cannot use it.
Using the ordinary GPL is not advantageous for every library. There are reasons that can make it better to use the Lesser GPL in certain cases. The most common case is when a free library's features are readily available for proprietary software through other libraries. In that case, the library cannot give free software any particular advantage, so it is better to use the Lesser GPL for that library.
This is why we used the Lesser GPL for the GNU C library. After all, there are plenty of other C libraries; using the GPL for ours would have driven proprietary software developers to use another—no problem for them, only for us.
However, when a library provides a significant unique capability, like GNU Readline, that's a horse of a different color. The Readline library implements input editing and history for interactive programs, and that's a facility not generally available elsewhere. Releasing it under the GPL and limiting its use to free programs gives our community a real boost. At least one application program is free software today specifically because that was necessary for using Readline.
If we amass a collection of powerful GPL-covered libraries that have no parallel available to proprietary software, they will provide a range of useful modules to serve as building blocks in new free programs. This will be a significant advantage for further free software development, and some projects will decide to make software free in order to use these libraries. University projects can easily be influenced; nowadays, as companies begin to consider making software free, even some commercial projects can be influenced in this way.
Proprietary software developers, seeking to deny the free competition an important advantage, will try to convince authors not to contribute libraries to the GPL-covered collection. For example, they may appeal to the ego, promising “more users for this library” if we let them use the code in proprietary software products. Popularity is tempting, and it is easy for a library developer to rationalize the idea that boosting the popularity of that one library is what the community needs above all.
But we should not listen to these temptations, because we can achieve much more if we stand together. We free software developers should support one another. By releasing libraries that are limited to free software only, we can help each other's free software packages outdo the proprietary counterparts. The whole free software movement will have more popularity, because free software as a whole will stack up better against the competition.