Between and during Emacs sessions, being able to view a list of my recently opened files via C-x C-r and then select and re-open them as needed.
;; recent files
(require 'recentf)
(recentf-mode 1)
(setq recentf-max-menu-items 25)
(global-set-key "\C-x\ \C-r" 'recentf-open-files)