Code happy: Find out what a company is really like
codehappy.info
codehappy.info
The tough part is that the ideal startup/job is different for so many people, and many of the things are subjective and hard to describe, so your best shot at getting quality data is to make sure you can encourage anonymous input, and then some sort of machine learning to turn all of that qualitative data into easily-digestible quantitative data.
Good luck and keep us posted on your progress!
I've already filled out a survey for my job and sent it to most of my developer friends.