Universal Texture Convertor V1, Particles 2, and MK Particles, PowerDials, and Radiosity scripts are a few I can't live without. (still on Poser 6 here)

Caro's free light set and Merchant utilities are a must-have as well (you can get them here, but I don't recall if it's for Renderosity merchants only - anyone know for sure?)

Ockham's Python scripts - gotta have 'em. :)

SkinShader is another one I use all the time, and RDNA's HDRI lights.