[octopine]

A JavaScript widget, browser plugin, or Photoshop filter to colorblindify, in various types, a webpage so that designers can check for usability would be useful.

EDIT: Actually, what would be more useful are colorblind OS X color profiles and an easy way to switch between them.

[macrael]

Xscope has a colorblind mode: http://xscopeapp.com/guide#screens

[__alexs]

Sim Daltonism http://michelf.ca/projects/sim-daltonism/

[aw3c2]

http://colororacle.org/