Tried Blender once (even had a hands-on session with some of the developers at some expo or other years ago) ... couldn't get to grips with it at all... never tried it since (then again, I can't even get along with 3D studio, so maybe the Blender interface isn't so crap after all
)
Still, plugins I find I can't (couldn't) live without are:
(1) fPrime ...
(2) Blochi's thickener - because I'm too lazy to do it by hand
(3) IFW Textures - because my texturing sucks and these have a nice collection to work with
(4) Any of the David Ikeda tools (before they were rolled into LW)