Lesser known built-in Emacs gems?

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