I have memorized most of La voz a ti debida, probably about half the Shakespeare sonnets. Most of Frost's sonnets. Almost all of Claudio Rodríguez. All the choruses from Henry V. Lorca? Yes, that too, though strangely enough he is not the poet I have memorized the most from. I think I did all his sonnets at one point.
I've done a lot of Williams. Many of the poems are short. I can still reel off many of them.
It is hard not to memorize poetry if that is your specialty. Of course, forgetting is also part of this process. I can probably say I've forgotten more than you ever memorized in the first place, unless you happen to share this particular method of study.
I got a complete set of Locus Solus, the legendary magazine of the early New York School, by purchasing the individual issues from various sellers. The collaboration issue, edited by Koch (#2), begins with a cento by Ashbery, "To a Skylark." I suspect it's all lines that he knew "by heart."