I've been exploring Emacs vanilla and found some gems. I'll list those and would like you to list as well in special those that you migrated to from a non built-in package.
fido: minibuffer completion
tab-bar: flexible tabs for frames and windows
desktop.el: saving/restoring sessions
rgrep: grep recursively
ibuffer: has filtering functionality
vc: frontend to git, mercurial, but very limited in comparison to magit
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