There's also a weird recent addition, the "Author-izer" [1], where the author can generate non-paywalled direct links to the definitive ACM DL version, and post

There's also a weird recent addition, the "Author-izer" [1], where the author can generate non-paywalled direct links to the definitive ACM DL version, and post those links on their personal website. This is distinct from the author-pays open-access option in that it's free, but just generates open-access links that you can post elsewhere, rather than making the paper fully open-access, as the pay option does. I don't bother to use it myself though, and instead just self-host a copy of the final PDF. [1] http://www.acm.org/publications/acm-author-izer-service http://www.acm.org/publications/acm-author-izer-service