Disney doesn't release that information so any such website would only be guesstimates. There are websites like Tour Guide Mike and UnOfficial Guide that use a method to predict how crowded each park will be on a given day based on (I think) what the crowds were like for that park on that day in years past. It's also based on things like if there is an EMH that day (or was the night before), and other special things that might be happening for that day. For that reason, the predictions tend to be changed once the official calendar of events is published (which used to be 6 months in advance but seems to be published a little later recently).
UnOfficial Guide I believe you can see a week or two of predictions for free, but you have to either purchase the book or pay a fee to get the rest of the year's predictions. TourGuideMike you have to pay to see this information. Many DISers use these to help them plan which parks to go to which days of their visit. Having viewed both of these websites, I can say that they have some helpful information....but if you want to save that money for use at WDW, all you really need to do is hang out here on DIS and listen to the "been there done that" folks. I made up our schedule based on DIS advice long before I viewed the websites....and I made no changes based on the websites after viewing them, but did appreciate the explanations of why DISers do what they do, lol....it made more sense after reading the websites, but only if you're one of those "I must know the why" types like me.
Hint....if you have the UG book you can view the website free....so either buy or borrow the book (hint hint library hint hint). I also borrowed Birnbaum's guide from the library....and again, found very little information in any of these sources that isn't available here on DIS. Of course, for the amount of time I spent on DIS reading/gleaning/planning, the books may have been a better use of my time, lol. But I also found very glaring errors in the books....including Birnbaum's saying it wasn't necessary to make ADRs for most restaurants (in the 2007 or 2008 edition).....ummm, following that advice would mean you'd be eating at a lot of resort TS, because almost everything in the parks is booked pretty tight even at not super busy months. Sure, you can often get a reservation last minute....but relying on that to get all your meals planned is really foolish advice in my opinion (and not practical during the busy times of year). We tweaked and changed our ADRs while we were there, but sometimes couldn't change things....so having something scheduled meant we didn't waste a ton of time trying to find somewhere to eat. And if you're on the dining plan, meaning you've prepaid for those TS, "settling" for a CS isn't an option. Of course it all depends on how you tour....but saying ADRs isn't necessary is bad advice for a book to give.
If you post when you're planning/thinking to be there, many DISers can give you some been there done that advice about crowd levels. The current threads about New Years Eve make me glad to have been home!