Google Scholar by now is the big player in free places to search across the literature. There are some oldies that are more specific in coverage: CiteSeer, DBLP, etc. And of course big government specialty: PubMed, Eric, TRID...
But now there are lots of other choices, which is mostly a good thing:
Microsoft Academic is back being actively developed. There's an API which allows some free usage and you can download the whole thing.
Base https://www.base-search.net/about/en/ - this has been around for a bit and people do really like it
Lens https://www.lens.org - also looks at patents
1findr www.1findr.com - has a free and a paid version. They have a commercial product to help you locate open access when available
dimensions https://app.dimensions.ai - also free and paid versions
As I say a lot, the problem isn't enough, it's probably one more of precision and efficiency. Also access to quality clean data in quantity.
Editing to add: Ryan Regier (also keeping an eye on the space and worth a follow) adds:
Science Open https://www.scienceopen.com/
Semantic Scholar https://www.semanticscholar.org/
Maybe also could add Mendeley - although it's built differently.