I use the same type pulleys as dd. I bought mine at a sailing shop, about $12 each. I use the 1/4 rope for both the tree and pack lifting rope. My set-up is a little different, as I use the Modification Azalea recommended. It just allows you to keep pack closer to one of the two tree's to eliminate almost all of the line sag.
I'm thinking of trying to use some of that 1/8 braided Dacron rope they sell at Cooke or Piragis outdoor. Holds 450 lbs and weighs .05 oz for 80 ft. Has reflective cord in it, so should make for some cool night time effects.  Supposedly is stiff enough for easy knot tying. I figure that for tree rope and keep the 1/4 for pack haul rope. Anyone have any experience with  the Dacron guy line rope?  Overkill weight wise, but would lighten the kit significantly IF practical?