[red rocket]

Stuff you couldn’t live without, some small hack that’s constantly running in the background, that totally transforms your user experience, the first thing you install, that you can’t do without?

Me, I have the following:

Smart Scroll ~ emulates iPhone scrolling behaviour
Auto Pairs ~ eliminates need for typing closing brackets, quotation marks, etc.
ProcessWizard ~ like Activity Monitor, except more powerful and sitting in the menu bar
GeekBind ~ Hotkeys for moving, resizing, fading windows

And Quicksilver, obviously.

[Curiosity]

MenuShade - dims down the menu bar when it is not accessed
Shapeshifter - allows coloured backgrounds to most windows instead of white
Mighty Mouse - changes the colour and shape of cursor - helps to make it visible against the background

[shifuimam]

Hey Folders! - emulates the old OS 8/9 behavior where colored labels actually color the folder
WClock - fab replacement for the clock in the toolbar (click the time and a calendar pops up; very handy)
FinderIconCM - because I like making custom icons
Micon - So i can easily use my custom icons
Witch - I like Windows' Alt+Tab behavior, so this does what I want more cleanly than OS X's implementation
Pulltab - so I can use Cmd+Tab with Witch (requires APE, unfortunately)
TinkerToy - so I can change dock behaviors and disable Dashboard

Then of course are actually apps I always install, like Firefox, Adium, Conversation, etc.

[zro]

The script menuextra along with whatever AppleScripts and shell scripts I'll be needing / wanting.