Collection of failed attempts to solve X
I've idly entertained the idea of an online arxiv overlay "journal" of negative results. The idea would be to allow negative results or counterexamples that don't seem suitable for a full publication in traditional venues. It would be lightly reviewed, so that we can guarantee that every paper has at least been looked over and approved by an expert in the area. It wouldn't necessarily be archival or "official", but it would at least be citable.

I think such a resource could be very helpful for publicizing dead ends in some lines of inquiry as well as "folklore negative results" that may not be widely known. I can think of a few examples from my own experience. A main challenge would be in finding and connecting with reviewers, due to the variety of possible research areas involved.

If there seems to be enough interest in this idea from the cstheory community, particularly people saying they'd be willing to submit and to review, then I'd be willing to take a lead in organizing. I'm pretty sure I can find some colleagues willing to join and help organize for the first few years.