Just now I thought of a question that I figured must have already been asked before, and indeed it was: here. However, it was a little harder to find than I expected. I would have expected that searching for "Ladner's theorem" or "NP-intermediate" would have turned it up, but neither did (at least not directly), even though those phrases do appear in the comments to the question. Is there a way to make the question easier for future searchers to find? Since it's community wiki, I guess I could just edit the question myself, but I'm not sure if that is appropriate etiquette. Also, since the search engine evidently doesn't work the way I expected, I'm not even sure what kind of edit to make to ensure the result I want.
Timothy, it is certainly appropriate to edit the tags and even the question if it improves the question in anyway, including making it easier to find in the future. Such changes, especially (re)tagging questions, is definitely encouraged, especially when you really know which tags belong with a question.
Changing a question (or answer) can be a little more controversial, if your changes are anything beyond fixing typos, spelling, formatting and so forth. But don't worry, if you do make a mistake that anyone is unhappy about, you'll hear about it fairly quickly.
I would have expected that searching for "Ladner's theorem" or "NP-intermediate" would have turned it up ... Is there a way to make the question easier for future searchers to find?
None of those terms appear in the body or title of the post; perhaps edit them in as appropriate, if you feel strongly searching for these terms should return that post.