Lambda the Ultimate

https://github.com/gasche/popl2017-papers/ has a list of POPL 2017 papers with crowd-sourced links to the preprints. The deadline for camera-ready version (a version provided by the researchers that takes reviewers' comment into account) is only on November 7th, so one should expect more preprints to be available after that.

Feel free to mention any paper that catches your interest. If you are very curious in a particular work that does not have an online preprint, sending an email to the authors to ask them whether they plan to have a preprint accessible online can help a lot.