So it turns out that for now there are no so many projects for extending LWC functionality. Possibly because the successor of Aura Components is nearly perfect ;)
What I have found so far:
A collection of easy-to-digest code examples for Lightning Web
Components. Each recipe demonstrates how to code a specific task in
the fewest lines of code possible, while following best practices. A
View Source link takes you right to the code in GitHub. From Hello
World to data access and third-party libraries, there is a recipe for
Not exactly an extension library, but contains many great examples of how to write our LWC code in a good-quality way.
This is a generic & customizable lookup component built using
Salesforce Lightning Web Components and SLDS style. It does not rely
on third party libraries and you have full control over its
It may come handy as there's still no standard LWC with lookup functionality.