Wonder no more. The Electronic Frontier Foundation (“EFF”) has launched the TOSBack website, which tracks changes to several different types of website policies, including Terms of Service, User Agreements, and Privacy Policies. The site's home page lists those policies with the most recent changes and the dates on which the changes took effect. You can click on each entry or on the specific website and policy to see a side-by-side comparison of the old version and new versions of the policy with the changes highlighted.
Go to the website for a full list, but it currently tracks changes to 44 separate policies including those of Amazon, eBay, Apple, Facebook, Whitehouse.gov and others. The EFF plans to track more agreements on the TOSBack site in the future, including agreements from credit card issuers, banks, and cable TV providers. Fair warning.