RUCA's - Correct excessive rear camber when lowering
TOE RODS - Correct out of spec rear toe from lowering
TRACTION RODS - Correct more bad geometry from lowering (this answer is a cop out as I'm not entirely sure what aspect of suspension geometry this one alters
)
TENSION RODS - Allows adjustment of front caster and replaces rediculous stock tension rod bushing for better feel while steering and braking
SUBFRAME COLLARS - Reduces wheel hop and general rear end stability problems cause by worn out stock bushings.
SUBFRAME RISERS - Compensates for changes in roll center caused by lowering.
Whether or not you need each piece is really dependent on your whole setup. Most people eventually pick up all of these except for the subframe risers. Those tend to be used mainly by the ultra slammed crowd.