What licenses do you use for your projects?

With all the hubbub around licenses lately, I wanted to kind of gather what licenses people use for different situations. I have been personally having trouble with finding a good license for a couple of my projects and wanted to see what others are using or would use for different situations.

My personal problem is that I want to let the users of my libraries/code to do whatever the hell they want with it, in terms of usage, linking, distribution, etc, as long as the code is in its original form. BUT if they make modifications to the code, I would like them to be required to share the changes and only to the library. I don't want their app code, or proprietary code.

I think I found the Mozilla Public License 2.0 is that, but I keep reading that it has clauses that aren't clear, and I am not great at reading legalese, so I haven't adopted it yet. and Apache License is amazing, but I want that weak copyleft. I wish I was good at legalese so I would write a new license to share.

I release 95% of my stuff as MIT/BSD 2-Clause, but for ones I want a little more strictness on, I want something. I wish some of the bigger licenses were as simple and clear as BSD licenses on the clauses.

So my question is:

What would you use for various situations? When you don't care what license do you use for your code? When you do care?

choosealicense.com has been somewhat useless to me for how often I see it recommended.