LMN Architect has just published a review of the DIVA plugin for Grasshopper.
DIVA uses Radiance, a free environemtal simulation software which you can download and install to run alongside Ecotect.
Radiance is more precise than Ecotect and can be used for Daylight studies, producing extremely accurate renders.
DIVA was written by Jeff Niemasz, a Harvard GSD student, as part of his masters project on re-visiting the aqua tower by Studio Gang.
You can register to the DIVA for Grasshopper group on the forum and follow the tutorial on installation (See videos below).[vimeo http://vimeo.com/29811576]
Above: Portfolio page from Green Re Articulates Red, my diploma project at the AA