I should have posted this a long time ago: the good people over at GMAT Club have created a GMAT score estimator. You can punch in your Manhattan GMAT, Kaplan, GMATPrep, and Princeton Review scores, and they'll give an estimate of your score on the actual GMAT exam, based on a model that they've developed. You can find the GMAT score estimator here. I'm not sure how well it works, but it's a brilliant idea. I've had far less luck with the GMAT score calculator created by 800Score.com. The idea is that you can punch in your raw scores from the math and verbal sections of the GMAT (on a 0-60 scale), and the website will calculate your composite GMAT score on a 200-800 scale. I've tried the calculator a few times using my students' raw scores from GMATPrep and actual GMAT tests, and the calculator always seems to be off by a little bit.
More recently, I punched in my own scores from the real GMAT. According to the calculator, a quant score of 51 and a verbal score of 47 will give you a composite score of 350, in the 7th percentile. Hilarious. Still a fun toy, but the execution could use some work.