Dropbox - Needs no explanation.
MacBiff - IMAP mailchecker.
Scroll Reverser - Flips Magic Mouse scroll directions, like Lion will.
SynergyKM - Use my iMac and MBP from the same mouse/keyboard.
System stuff: Volume, Airport, VPN, Power, Date/Time.
- Calendar applet.
System: Spotlight.
Note: This is my MBP, which I use the most. My iMac and media-server mini are different.
(From Brad Beyenhof)