What are your favorite menulets (or Menu Extras as Apple calls them)? Yes, I know, it's another list, but here goes:

What is your favorite menulet and what is it used for?  


Please one post one menulet per answer.
Add a link to the developer's website, if possible.
Duplicate answers should be flagged for deletion, so do check or edit an existing entry rather than adding a second answer for the same software.
Caffeine

Caffeine is a tiny program that puts an icon in the right side of your menu bar. Click it to prevent your Mac from automatically going to sleep, dimming the screen or starting screen savers. Click it again to go back. Right-click (or ⌘-click) the icon to show the menu.