Sorry to be late in the party, but I think there was some confusion in the first message of this conversation. Open Source and copyleft are defined using copyright and all Open Source Software (OSS, aka FOSS) is copyrighted. The people that created the work own the copyright and by licensing their work with Open Source license they've allowed others to use, modify, learn from and redistribute (share) their work. That doesn't mean they've given up their copyright to their work.
It is quite common practise for Free and Open Source Software (FOSS) game developers to use (L)GPL for code and Creative Commons (CC) licenses for artwork (textures, maps, models, etc.) so we could use this ourselves too. CC has a few options that restrict the distribution of game assets in different ways. The most permissive is CC-BY only requires attribution and most restrictive is CC-BY-ND-NC which requires attribution and doesn't allow derivative works (changing) or commercial use.
Copyleft licenses like GPL don't allow others to change the license of their work and require the four essential freedoms from all derivative works, but permissive licenses allow even redistribution of the work under other licences as long as their clauses are fulfilled. Some popular permissive licenses: MIT, BSD (or the other BSD) and Apache licenses. Those are also quite a bit shorter.
Creative Commons has made this nice chooser page to aid license selection: https://creativecommons.org/choose/
About Open Source Software in general and lots of licenses: Open Source Iniative
They also maintain the Open Source Definition.
Remember that even if you release something under some license, you still hold the copyright and can release it again under some other license. Also if you really want to give up your copyright (which may not be possible everywhere around the world), you may want to either say it is public domain or, even better, use CC0 license which effectively does the same in the limits of local copyright laws.
When I say OSS, Free Software or FOSS (combining both) I mean the same software, since, for our purposes they are the same. The terms have different backgrounds: OSS is more about the software engineering point of view and Free Software is more about the freedoms of the users. Free Software was here first and OSS came when corporations got interested in Open Source and its benefits to software development.
I would say that use any Open Source license. If you like, use Creative Commons for the assets. Personally I would go with either GPL or MIT for the code and some combination of CC-BY, SA (Share Alike) and NC (Non-Commercial) for the artwork and such depending on what I want (probably CC-BY or CC-BY-SA).
Basically any Open Source (Free Software) license for the code and whatever Creative Commons that fits for the other things (it can be very restrictive, but still allow redistribution).
I suppose the most restrictive Creative Commons would work here. Although it's not meant for source code, but in this case it should work.