These applications and add-ons aren't necessarily the biggest productivity boosters; they don't have the most features or options. What qualifies them to be included in this list is that they all (as they say in Macdom) simply work. They are all perfectly seamless, and just feel like part of the system...
Uno brings the unified look to your interface
Butler lets you create menus and shortcuts to literally everything
CandyBar lets you change your default icons
Witch For Better Slicker Switching
Sogudi adds power to the address bar
Menu Master is a quick way to modify keyboard shortcuts
FolderGlance lets you stay here while moving things there
TinyAlarm is an easier way to set alarms
Tofu is a clean looking text viewer
ClearDock for tweaking your Dock's background