The New York Times took the time to analyze and compare the different credit scores from each of the three credit bureaus and other websites where you can obtain your "credit score". I use quotes because only some of the sites actually provide a score that is likely to be used by a lender.
For instance, the three credit bureaus each offer to sell consumers their credit scores. But only Equifax uses a score that is actually based on a scoring model used by potential lenders. Experian and Trans Union's scores are thus less useful to consumers than the score sold by Equifax.
Also the congressionally required site http://www.annualcreditreport.com/ uses a FICO score, which is used by lenders. You can also get this score via FICO's website. But the credit score offered by http://www.creditkarma.com/ does not use a scoring model used by lenders. The website http://www.quizzle.com/ is not mentioned in the article but also does not use a score based upon a scoring model used by lenders.
The full article is here - http://www.nytimes.com/interactive/2009/11/29/business/scores2.html.