If you use multiple monitors, and you're frustrated with the cmd-tab app switcher only appearing on the last monitor you touched the dock, this will make it show up on all monitors.
defaults write com.apple.Dock appswitcher-all-displays -bool true
Option-Command-C to copy the path of a file to the clipboard (if you need to use it somewhere that doesn't support dragging to paste a path).
if you have a file dialog open, hit Command+Shift+G to open a dialog that let's you type in the path.
defaults write com.google.Chrome AppleEnableSwipeNavigateWithScrolls -bool FALSE
defaults write com.apple.finder AppleShowAllFiles YES
defaults write com.apple.dock no-bouncing -bool TRUE
Cmd+Shift+Period : toggles 'shows hidden files' in Finder
Unclutter - This gives you a nice non intrusive "memo zone" as well as manage your clipboard, so that copying password won't lose your user name into limbo.
Smart Folders (Finder -> File -> New Smart Folder). It's not exactly hidden but I never paid too much attention to it. It's essentially a way to create a folder whose content is dynamically updated based on your search conditions.
cmd +b from a spotlight search == browser search
Finder: Cmd+Shift+G to navigate wherever I want (with autocomplete)