Powerprep probably comes the closest. i wont trust kaplan
and princeton scores.
We do have a score estimator and it does the predicting fairly well. you will see the estimator in the Resources section on the website.
In general, practise tests will not help too much in predicting scores simply because its too difficult to replicate the stress and the level of preparation you will have on test day.
What we did was to ask guys to input their practise test scores and their real gmat score into the database. We have 250+ entries in the database now. what this does is establishes a fairly ok relationship between the practise test scores and real gmat. but again, the score estimate you will get is just that, an estimate.
hope this helps