The CTIA today launched KnowMyApp.org, a site detailing the average data usage of popular phone apps.
The service currently includes the top 50 apps for both iOS and Android. For each, it describes a tested scenario (designed to be typical) and shows how much data it used. It also shows what percentage of common data plans (1, 2, or 4 GB) the app could be expected to consume.
The service is useful not only for avoiding data caps, but for maximizing battery life, making the service useful even for those on unlimited data plans.