Eval.google.com is another hidden domain from Google…perhaps used for testing purpose but people found it out. It has been termed as Google’s Secret Evaluation Lab as followed from the domain name. And that it is being used for spam control plus the employees working on it are either students or part-time employees.
You might like to check detailed view in this Flash file.
And if you are a google employee, you might get more insight into it by going to – https://www.google.com/evaluation/search/rating/task-edit?task=XXXXXX.