The rr debugger [1]. I desperately missed it after switching to a mac laptop -- to the point that I now have an older linux system which exists primarily to run rr.
(I've heard that Pernosco [2] - partly built on rr, AFAIK - is even more revolutionary, but haven't yet tried it myself)
FYI, Jean Yang had Robert O'Callahan on her weekly PL-themed "talk show" on Twitch to talk about rr a couple weeks ago. The video is available until this coming Friday, I believe [0], at which point it will disappear into the ether.
(I've heard that Pernosco [2] - partly built on rr, AFAIK - is even more revolutionary, but haven't yet tried it myself)
[1] https://rr-project.org/ [2] https://pernos.co/