Kenny the Pirate (Free) and TouringPlans(not free, but really is $15 going to break your multi-thousand dollar Disney vacation?).
They both give good guidance as to which parks will be the optimal each day of your stay. A couple of general themes that I've discovered over the years to keep your crowd levels low:
1) Stay away from EMH evening days
2) Avoid MK on Mondays
3) Avoid Epcot on Friday and Saturday nights during Food and Wine festival.
Touringplans has so much more to offer than just crowd predictors. It has photos from every room of every Disney resort, which really helps with room requests. There is an automatic room request feature. It also has a dining reservation finder, which I have found to be VERY valuable. It continually searches for your desired ADR and sends a text and/or email when it finds it. (you need to act quick to snag the ADR though). Dining menus, touring plans for each park, etc... No I don't work for touringplans but I do place high value on having a subscription.
Dan