Interestingly, I don't remember when the last time I did so was. I seem to be relying on SO less and less. I do sometimes use it as a quick reference, usually because its one of the first results on search engines, but its typically just a quick "ah, that's the function for that" or "ah, that's how you do X" rather than actually copying code. I'm trying to think what's changed, I guess I've just been writing stuff th…
Most frequently I do it for PHP. I help a friend with a Wordpress site, PHP is easy to read but I'm hardly fluent in the ins-and-outs, and a simple Wordpress hook added to functions.php can fix so many issues. I always leave a comment with the full link for context.
I always leave a comment trying to explain to my future self where I learned to do stuff that was not intuitive.
Bash, Make, non-standard Python libraries, E-Lisp, and Fortran.
Also, it's been quite a while since I've touched JavaScript, so the newer features have been harder to grok.