One handy resource I use is http://www.appannie.com
It’s useful for getting daily global chart positions of games that have been out for a few months. And that part is free. You can get more info by subscribing.
There are other sites like this out there but they seem to only offer you a few months free access before asking you to sign up and pay. Useful if you want to find out how a particular company or publisher is faring. It’s a step up from counting how many Facebook follows a company has to gauge it’s popularity.