You're welcome! Folding@home is the big one, and the Great Internet Mersenne Prime Search is also pretty popular (though IMHO a waste of resources for a relatively useless result). But I just looked into this topic myself after posting that comment, and turns out there's a huge list of such "volunteer computing" projects: https://en.wikipedia.org/wiki/List_of_volunteer_computing_projects

So while Folding@home is a great one and medical scientific research, you might pick something else from that list. Perhaps more than one!

Now the confession: I'm a hypocrite. I never ran any of these volunteer computing projects on my own PCs. But that's partly because I tend to shut them off every night, so a lot of the usable time for it isn't really usable. The other part is basically that I never bothered to do it.

But I think after this conversation reminded me of it, I might look into installing it on my PC!

You should check out the great internet mersenne prime search as well: mersenne.org

Edit: forgot to read comments first