smcFanControl - To make sure that my old Macbook is not gonna stop brutally due to those annoying Websites still using Flash.
Dropbox - A must have.
TextExpander - I mostly use Markdown, HTML and date replacement snippets.
Cloud App - Very neat tool to easily upload screenshots and files.
Hazel - Great tool to rename and sort files in my Downloads folder but also to sort my Dropbox files when they get automatically on my computer thanks to services such as SendtoDropbox or DropItTo.
SlimBatteryMonitor - A neat app to replace the default Mac Battery icon, I only need to know the percentage of remaining battery, not the logo. Besides I am usually already aware of the fact that my battery is charging thanks to the red light on the charger. 
- To keep a log of my copied items.
Caffeine - Very convenient when Skyping or watching videos in a browser.
Sound, Bluetooth, Wi-fi, Clock and Spotlight - The basics. I have the Bluetooth there only because I use a Bluetooth earset.
(From Louis)