Mod Content Usage
Using content from other mods
It's a sad fact, but in some cases content is also being lifted from one mod to another. It's often a misconception that just because a mod is free to download, its contents are also free for you to take and choose from as you wish.
Even if a mod is free or not, the creators of any original work included within it (sounds, materials, models, source code) own the copyright for their work. This is true if you are a high-school kid, a corporation, or just some other modder who puts stuff together for fun. Every author gains these rights by default when they create their work.
The authors alone can decide how and who may use their work and have the law behind them should they decide to take issue of anything used without permission. If you see something in another mod that would be of use to you in your own mod or map, your first step should be to contact the mod team and then specifically ask the creator of the item your after. They and they alone can give you permission to use their work.
If you create contents for mods and don't mind other people using your content, then it could save a lot of trouble and benefit the community as a whole if you release your works under a Creative Commons license. See the Official Website to select a License for your works.
Using content from other games
You can access any number of games through Steam, even if they don't use Source, by tweaking your gameinfo.txt. This doesn't require extracting anything from a GCF, but you can't guarantee that the user owns the games you've mounted.
Legally-speaking however, making (for example) a Source mod that features music from Defcon may not be permissible, even though only those who own Defcon could play it. This is because there are two competing rights:
- The right for the creator to dictate how their works are distributed
- The right of those granted access to works (e.g. by buying them) to mix them together for personal use
There is no legal precedent for the quasi-authorised content mixing that Steam allows.
Games in general
Simply don't do it without a proper permission.
The copyright discussion above is just as much in effect with commercial games, and you are also under an End User License Agreement(EULA). EULAs usually forbid decompiling/modifying/re-distributing the game's content.
Also if your mod has a forum where users can post models, make sure they don't post copyright infringing content. Under the Digital Millennium Copyright Act (DMCA), once a web host finds/is notified of infringing material on their site, they are obliged to remove it or they must face liability for copyright infringement.
So if you let your users post infringing content (such as content from other games), not only the users but you can be accused.