See Also: Libre Game Dev Tools
Game assets which should be usable for DFSG-compatible open-source games.
(i.e. assets which were either released into the public domain or were offered under a license compatible with the terms of the Debian Free Software Guidelines at the time I downloaded them and, thus, even if the creator changes their mind later, access remains under the old terms via anyone who had already acquired them and wants to pass them on.)
There's a lot of legal ambiguity in casual "do what you want" license terms and "no redistribution" terms mean that, if a site goes down, all those resources are lost or legally poisoned forever. (Given how copyright keeps getting extended.)
The requirements of the Debian Free Software Guidelines were crafted based on specific concerns about how a bad actor could make people miserable or cripple their competition or how a disaster could do likewise.
For example, Debian checks for things like license non-revokability via a thought experiment they call the "Tentacles of Evil" test because it's intended to protect against an evil corporation or government coercing or buying out the creator and then making life miserable for people using what they created.
At the same time, the "Desert Island" test disallows "no redistribution" restrictions, because we don't want your game assets to die with you or the site you posted them to, and "no reselling" is also disallowed because all three big definitions of free/libre/open-source software (FSF, OSI, Debian) agree that "restrictions on fields of endeavour" aren't OK.
(Abusive behaviour is instead mitigated by requiring that you give credit to the creator and, if it's a concern, by using a license like the CC-BY-SA or GNU GPL which requires modified versions to be under the same terms. Thus, ensuring a free supply of the specific icon/background/etc. to compete with the bad-faith actors trying to sell it while allowing good-faith actors to do things like selling Wikipedia on DVD to schools in developing countries with poor Internet access for some token "cost of labour and materials" fee.)
Also, personally, I see inclusion in curated Linux package repositories like the Debian package repository as a great thing for a free game to aspire to. If your entire game isn't under DFSG-compatible licenses, you'll have an uphill battle on your hands to get it into the default package repositories for Linux distributions, since they reserve their "non-free" sections for things they carry under protest but can't avoid, like "gratis but not libre" firmware blobs needed to make your hardware work.
Examples of compatible licenses include the CC-BY and CC-BY-SA but not the ND (no derivatives) or NC (non-commercial) variants. Releasing things into the public domain is also fine, but the CC0 public domain dedication is preferred because it includes a fallback to an equivalent license in jurisdictions like Germany where you're not allowed to prematurely give up your copyright and you have to cleverly work around "you can't pre-emptively give up rights that get written into law later on".
NOTE: The Free Software Foundation has a good explanation of why informal "do whatever you want" licenses aren't safe for inclusion in this list. If it's not under a proper, formal license at the time I look at it, it's not getting on this list.
Yes. Many popular indie games have used libre assets.
If the assets are in the public domain (eg. CC0), then you can do whatever the heck you want, but don't be a jerk. Give credit anyway. (Covering your behind this way will also improve the chances that unbalanced people will go after the original asset creator instead of you.)
If the assets are under a "permissive" license like the CC-BY or the MIT license, all you need to do is give credit to the author in the form they specify. (eg. If they want a link to their site, you have to include a link or URL in your credits rather than just their name.)
If the assets are under a "copyleft" license like the CC-BY-SA (Don't worry. Almost none in this list are.), then you want to make sure that your game is seen as an "aggregation" of the libre bits and the bits you don't want people to be able to copy without paying, rather than a "derived work".
(A "copyleft" or "hereditary" license is a license where, if you modify something, the modified form has to be shared under the same license but, suppose you want to put a bunch of resources in a Zip file for a friend. The law doesn't suddenly say that everything else in the Zip file is "derived from" the copy of GIMP that you added.)
For example, if you want to use a libre icon set that's under a copyleft license:
...and, no matter what license it's under, if you like what you got, try to give something back. (Whether it be a donation, releasing some libre assets of your own to pay it forward, or even just spending more than the base minimum effort helping to get the creator some publicity.)
Old version is available for free.