Most have been mentioned, but, hey, I'll mention them again since they are ones I use also, but not in any particular order.

A clothing Convertor, such as Xdresser or Wardrobe Wizard for character fits.
Morphing Clothes by Dimension3D.
VSS and Environsphere and everything else by Bagginsbill. I wish all vendors used parmatic to help adjust they're characters skin, but that's why we have VSS.

ShaderSpider. I don't use it as much as I used to, but it always saves me time when I do use it.

Poser File Organizer by Dimension 3D to fix broken files and paths, plus it comes with a handy .rsr to .png convertor

There are a bunch of handy new python scripts inside Poser 8, which I haven't had time to fully explore. Semideu's Light Control python is handy, so is D3D's new Render control.

Renderosity's Poser Forum Material Room stickied Thread -- A MUST READ and very handy.

Ockham's Delete all morphs  python is one I use regularly; just wish I didn't have to it set up in advance to use it (I have all my characters listed, but to do it for each clothing item is a PITA).

The newer Python Scripts pull down menu is fantastic.

I also like the morph brush tool. Haven't had a chance to try out it's improvements in P8 yet, though.

Well... mostly python scripts :)

-- Jeff