The Consumer Financial Protection Bureau now has an online tool
to estimate Social Security benefits:
http://www.consumerfinance.gov/retirement/before-you-claim/
It's only a rough estimate, but it seems to be close enough for general planning purposes,
and it illustrates the differences in benefits at various retirement ages.
(There's also a link to a good Social Security Primer on my website -
it's on the main menu.)
As always, consult with a qualified financial advisor before making any decisions!