I’m developing a software package and would like to make it (and the source) available for personal, academic, or educational use, but not for commercial use. Is there such a license? Ideally, it should be reciprocal, so that if someone publishes a modified version, they’ll have to grant the same rights.
Alternatively, would it be possible to modify the GPLv3 in such a way that it forbids commercial use and grants rights only to non-commercial users?
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I know of a project (omnet++) that does something similar. The license gives permission for personal, academic, or educational use, but not for commercial use: https://omnetpp.org/intro/license
They say that the license is kind of similar to the GPL.
ACADEMIC PUBLIC LICENSE
…This license contains the terms and conditions of using OMNeT++ in noncommercial settings: at academic institutions for teaching and research use, and at non-profit research organizations. You will find that this license provides noncommercial users of OMNeT++ with rights that are similar to the well-known GNU General Public License, yet it retains the possibility for OMNeT++ authors to financially support the development by selling commercial licenses. In fact, if you intend to use OMNeT++ in a “for-profit” environment, where research is conducted to develop or enhance a product, is used in a commercial service offering, or when a commercial company uses OMNeT++ to participate in a research project (for example government-funded or EU-funded research projects), then you need to obtain a commercial license for OMNeT++. In that case, please contact the Author to inquire about commercial licenses.What are the rights given to noncommercial users? Similarly to GPL, you have the right to use the software, to distribute copies, to receive source code, to change the software and distribute your modifications or the modified software. Also similarly to the GPL, if you distribute verbatim or modified copies of this software, they must be distributed under this license.
By modeling the GPL, this license guarantees that you’re safe when using OMNeT++ in your work, for teaching or research. This license guarantees that OMNeT++ will remain available free of charge for nonprofit use. You can modify OMNeT++ to your purposes, and you can also share your modifications. Even in the unlikely case of the authors abandoning OMNeT++ entirely, this license permits anyone to continue developing it from the last release, and to create further releases under this license…
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Would it be possible to modify the GPLv3 in such a way that it forbids commercial use and grants rights only to non-commercial users?
No.
The GNU General Public License is itself a copyrighted document, and is not available under free license terms:
Copyright (C) 2007 Free Software Foundation, Inc. http://fsf.org/
Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.
You do not have permission to take the license and tweak it to your needs. You can model a new license on it, but it would need to be a ground-up rewrite, not a simple modification of the terms of the GPL.
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You could take inspiration (with the help of your lawyer) from the license of CompCert which is not an open source software.
BTW, many academics won’t even look inside such a software (even if the license permit them to do so).
NB. I am not a lawyer!