Duplicate content or pages with similar content is a major problem on the web both for users and for web designers and SEO specialists.
There are many tools around the web to help alleviate the problem. These are my favorites:
Webconfs Similar Page Checker after entering the 2 urls outputs the results as a percentage.
Duplicate Content Tool (no longer available) Shows some interesting data like Total HTML similarity, Standard text similarity and Smart text similarity which analyses the main content of the page stripping out the footer, navigation, etc.
ilCaimano Page Compare tool, in it’s current version still in beta compares the Html and the text profile (with the HTML stripped out).
Another tool (aptly named “Similarity Analyzer” ) that I used to use along time ago is from an Italian site called tool.motoricerca.info