Some examples:

- https://www.academiclabs.com/
- [Open Science Framework](https://osf.io/)
- https://www.scilifelab.se/: Open for collaboration: an academic platform for drug discovery and development at SciLifeLab (https://doi.org/10.1016/j.drudis.2016.06.026)
- https://www.kaggle.com/ to some extent can be used to find academic collaborations centered around a given data set ([example](https://www.kaggle.com/allen-institute-for-ai/CORD-19-research-challenge/discussion/136125)).

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https://researchgate.net was mentioned in some other answer, but it is very scammy, see:

- [Should I send a “cease-and-desist” letter to ResearchGate?](https://academia.stackexchange.com/q/24127/452) 
- [ResearchGate: an asset or a waste of time?](https://academia.stackexchange.com/q/16870/452)

https://www.academia.edu/ also has their own issues, e.g.:

- https://en.wikipedia.org/w/index.php?title=Academia.edu&oldid=945557467#Criticism
- [Why does academia.edu reserve the right to sell, modify, and “exploit” my papers if I post them there?](https://academia.stackexchange.com/q/16050/452)
- [Is Academia.edu useful?](https://academia.stackexchange.com/q/44632/452)