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
For non-commercial mods and fangames, assets directly from first-party Valve games such as models, textures, materials, sounds, dialogue, and even music may be ported between Valve games or ported to other games (including those on non-Valve engines).
This does not include game code (which is subject to stricter SDK licenses), and vanilla maps should generally not be ported verbatim without decompiling and editing.
Additionally, content packed into community-made Workshop maps that are featured in games such as 
Team Fortress 2 or 
Counter-Strike: Global Offensive should not be used unless the author permits it.
For commercially released games, Valve should be contacted beforehand. Textures and materials directly from first-party 
Source engine games may be used in commercial games running on Source (but not ported across engines), as long as they do not directly contain Valve trademarks. Other usage requires prior authorization.
Assets from third-party games using Valve's IP is subject to separate licenses. For example, Nexon does not take kindly to usage of assets from 
Counter-Strike Nexon: Studio in other games. Usage of assets from any game not listed on Valve#Products is subject to separate licensing rules than those for first-party Valve games.
See the GameBanana porting whitelist for loose guidelines on content usage of other company's game assets.
Through Steam
You can access content from 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 VPK, but you can't guarantee that the user owns the games you've mounted.
Note:If you are thinking of extracting content from a VPK and distributing it yourself, see the next section.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 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. The safe harbor clause of the DMCA generally only applies to ISPs, not web hosts, so if you let your users post infringing content (such as content from other games), not only the users but you can be accused.