Thanks for the replies W00tf0rfr00t, Mike and rnlf!

I will probably go for an open source (according to the OSI) code base, licensed under GPL, and have a few add-ons/plugins, also "open" (i.e. code viewable/modifiable) under a different license:

- Free for academia
- non-free for commercial applications

This way I would also benefit of being able to be categorized as " OSI open source" in listings, but still make different conditions according to the user (Academia vs commercial)

I actually have a last doubt/question:

Say, I make my application open source under GPL. Can I still grant some special rights to some people/companies, so that they can release their modifications under a proprietary license?
In my opinion yes, since I am still the owner and copyright holder of the software.

Exactly. You own the copyright. You may provide any exceptions you want. But be aware that it will be hard (read: impossible) to enforce stronger restriction on some people, because they will then always switch to the open license.

Well, there is NPOSL, which I guess is as close to "academic" as you can get (assuming that "academic use" makes no profit).

Though I wonder what the hindrance is to doing the same as a thousand existing projects: Dual license GPL and proprietary license. This
a) allows academics to use the work (under the terms of the GPL, which is not a problem for academic work)
b) allow hobby programmers to do the same (under the terms of the GPL)
c) does not allow companies to exploit your work, except if they release their product under the GPL as well (which is usually not an option)
d) allows companies to exploit your work after paying for that right