these are apps on my 4
winterboard (themes etc)
sbsettings
rotation inhibitor
auto ssh (so i can access system files)
no camsound (no shutter sound)
mywi 4.0 (make your iphone 3gs or 4 a wi-fi hotspot)
backgrounder (i hate iOS4's multitasking)
my 3g (tricks your phone, think it's on wifi)
locktopus (puts a passcode on apps you wish to make private)
i have more apps from cydia but these are the most important since i love changing the way my 4 looks