Sophos Anti Virus - Because you can never be too careful!
Fantastical - I use this to simplify my iCal workflow, especially for school. 
smcFanControl - This keeps my Mac cool during processor intensive activities. I have several preset speeds that I choose based on what I’m doing. It also boosts the fan speed by 750RPM while my Air is charging. 
Ejector - I use this especially on my MacBook Air to make removing my external disks simple. 
FreeMemory - I have this set to automatically purge my inactive RAM whenever it drops below a certain level. Keeps my Air smooth and responsive. 
Dropbox - Self explanatory. Also, awesome.  
BwanaDik - Keeps tabs on my current Wi-Fi situation. More informative than the built-in menu app. 
MenuPrefs - Simplifies changing the System Preferences. 
- Apple should have done this…
CloudApp - For quick sharing of small items between my Macs and friends
Evernote - For jotting down stuff I want to remember
- Displays the weather in really nice HD graphics, and the menu bar app drops down a list of current temps for selected cities. 
Caffeine - Keeps my Mac running high. 
Then the standards: Keychain Access, Time Machine, Bluetooth, Battery, Wi-Fi, Volume, Date/Time, and Spotlight.
(From Vincent Sanchez)