What are your favorite menulets (or Menu Extras as Apple calls them)? [closed]

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.

Screenshot

iStat Menus

by Bjango is definitely my favorite. Monitor CPU, memory, disk usage & activity, network, temp sensors, battery, and customize the date & time format. Each of these items also has a customizable drop down menu with more details.

iStat Menus preferences

iStat Menus in menubar

iStat Menus memory monitor

Fantastical ($15)

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Fantastical is awesome for quickly creating new calendar events and for seeing what's coming up. It's got some pretty smart natural language recognition, so I can just type "Architecture Review Meeting next Thursday from 4 to 6pm in Conference Room #42", and it fills out all the fields for you. Hit return and you're done!

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MenuMeters

MenuMeters is a free application that displays CPU usage, memory usage, network up/down, and disk write/read.