Some examples:
- https://www.academiclabs.com/
- Open Science Framework
- 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://researchgate.net was mentioned in some other answers, but it is very scammy, see:
- https://en.wikipedia.org/w/index.php?title=ResearchGate&oldid=947542863#Criticism
- Should I send a “cease-and-desist” letter to ResearchGate?
- ResearchGate: an asset or a waste of time?
https://www.academia.edu/ also has their own issues, e.g.: