A simple way to see what those files in your Trash are before consigning them to the great digital beyond. Quick Look works on trashed files and folders, so you don't have to drag items out of the Trash to double-check 'em.

(This is nice because otherwise you have to drag items out of the trash before opening them, a hallmark of the Mac for quite longer than I can remember.)

[robg adds: Actually, an older hint here pointed out you can drag them onto an already-running application icon in the Dock to open them.]