Here are some shortcuts in OS X that I find useful.
§ 1. Keyboard shortcut
Show Character Viewer. Before OS X 10.9 Mavericks:
alt + command + tthe Character Viewer window will show up.
control + command + spacethe Character Viewer shows up as a pop-up bubble. Type directly to search.
§ 2. Folder locations
Default wallpaper (desktop background) folder