Pick three...

Stinkie

PushPull
the Line Tool
inferences (yes ... it's a tool )

remus

Inferencing
Organiser
Skindigo

All things that are hugely useful, and for the most part under used.

rcossoli

that difficult, with so many tools available among the tools that brings the program and other developers who made it is difficult to choose only 3.

Push / Pull
Follow me
Kerkythea

plot-paris

Inferencing*
Push/Pull
3.??? not sure yet (there are so many)

just recently I had a go with several different modeling applications. damn, I missed the inferencing! usually the programs had some sort of snap modes. but they all felt like a workaround. Inferencing, I love you!!!

solo

Push pull
Podium
Subdivide and smooth

Daniel

It's impossible for me to pick only three, as I use so many. However, the tools that probably help save time and resources are (in no particular order):

create component/group
inferencing
push/pull
follow-me

dale

inferencing
push/pull
scenes

at least for the moment

plot-paris

oh yes, Daniel. Groups/Components and the easy, intuitive way to navigate within nested structures is such a great way to organize a model! the system is so simple and yet so powerful; unlike any other software I know

dale

Damn, I should have chosen eraser

skaught

inferencing
push/pull
and ummm...is this forum a tool?
if not... then I guess the pick tool has been particularly helpful.

edit: ok I thought about it some more...

inferencing
intersect selected
push/pull