Google has added a new way for webmasters to tell it which parameters in URLS, they wish to be ignored. They have added a new feature to Google Webmaster Tools called simply, "Parameter Handling." Google provides the following explanation with the feature:
Dynamic parameters (for example, session IDs, source, or language) in your URLs can result in many different URLs all pointing to essentially the same content. For example, http://www.example.com/dresses?sid=12395923 might point to the same content as http://www.example.com/dresses. You can specify whether you want Google to ignore up to 15 specific parameters in your URL. This can result in more efficient crawling and fewer duplicate URLs, while helping to ensure that the information you need is preserved. (Note: While Google takes suggestions into account, we don't guarantee that we'll follow them in every case.)
The feature is yet another option webmasters can use when trying to eliminate duplicate content issues, which as we all know can be harmful to rankings, even though Google says it's not a penalty. Either way, eliminating duplicate content when possible is likely to be in your best interest.
To use the feature, just go to Google Webmaster Tools, click on site configuration, and settings. There you will find the "parameter handling" option.
Barry Schwartz at Search Engine Roundtable notes that Yahoo has had a similar feature for quite some time, which it calls the "Dynamic URLs" feature. Ex-Googler Vanessa Fox has a very informative piece on the topic of URL parameters available here.