While 'asset flips' are becoming a common occurrence, pre-made art assets are still a valuable resource when used correctly. Asset Forge and Kenney assets might be quite useful!
Asset Forge [Official Site] sounds like a really clever tool. Like sandbox games where you build using blocks and stick them together, Asset Forge works in a similar way with it giving you a bunch of pre-made blocks to stick together to make assets for games. The great thing about it? The editor has a Linux version! For people like me who love to tinker and create, but have zero artistic skill it sounds awesome.
Once finished, you can export your creations for use in many different game engines like Unity, Unreal, Construct and plenty more. They can export to PNG, OBJ models and FBX models.
It's created by Kenney [Official Site], who actually make fully pre-made assets either for free or for a cost. What makes me really quite like their work, is that there's no strings attached as you get them under a public domain license. Here's an example of the quality of what they offer, this is from the Space Kit which is under the CC0 1.0 Universal (public domain) license:
Kenney have a bundle of their assets for sale on itch.io currently too. With 13% off three asset bundles, it will give you thousands of assets to use however you want, since they license their creations under a Public domain license. It seems the packs of assets they sell, include a lot of resources that are unique to each pack, but still you get the freedom to do with them as you please.
Next time I want to tinker with a game engine, I will probably look to some of these to play around with. Their stuff looks good and it's a really great license choice.
Hopefully some of you might find these useful.